Inclusion Criteria:

   - Subject with idiopathic hypereosinophilic syndrome must meet the following:

      - Has as at least 2 readings with an absolute eosinophil count >= 1,500/mm^3 in the
      preceding 3 months prior to starting ruxolitinib (one reading must be during the
      screening period).

      - Dependent, intolerant or refractory to corticosteroids OR has relapsed/refractory
      disease to other therapy besides corticosteroids.

      - Symptomatic from his/her disease OR has one or more signs of organ damage
      (assessed by the investigator as possibly-related to eosinophilia or
      biopsy-proven). This can include skin, lung, cardiac, central nervous system,
      liver, or gastrointestinal (GI) involvement, or evidence of symptomatic hepatic
      or splenic enlargement.

   - Subject with lymphocyte-variant hypereosinophilia must meet the following

      - Has at least 2 readings with an absolute eosinophil count >= 1,500/mm^3 in the
      preceding 3 months prior to starting ruxolitinib (one reading must be during the
      screening period).

      - Dependent, intolerant or refractory to corticosteroids* OR has
      relapsed/refractory disease to other therapy besides corticosteroids.

      - Symptomatic from his/her disease OR has one or more signs of organ damage
      (assessed by the investigator as possibly-related to eosinophilia or
      biopsy-proven). This can include skin, lung, cardiac, central nervous system,
      liver, or GI involvement, or evidence of symptomatic hepatic or splenic
      enlargement

      - Has abnormal T-lymphocyte immuno-phenotype by flow cytometry.

   - Subject with chronic eosinophilic leukemia, not otherwise specified (CEL,NOS) must
   meet the following

      - Has at least 2 readings with an absolute eosinophil count >= 500/mm^3 in the
      preceding 3 months prior to starting ruxolitinib (one reading must be during the
      screening period).

      - Newly-diagnosed OR receiving corticosteroids OR has relapsed/refractory disease
      to any therapy besides corticosteroids.

      - Has increased blasts in the blood or bone marrow (> 5% and < 20%), and/or a
      clonal cytogenetic or molecular abnormality

         - Subjects with JAK2 mutations are included within this group.

   - Subject with JAK2-rearranged eosinophilic neoplasm must meet the following

      - Has at least 2 readings with an absolute eosinophil count >= 500/mm^3 in the
      preceding 3 months prior to starting ruxolitinib (one reading must be during the
      screening period).

      - Newly-diagnosed OR receiving corticosteroids OR has relapsed/refractory disease
      to any therapy besides corticosteroids.

         - This group includes subjects with PCM1-JAK2, BCR-JAK2, ETV6-JAK2 or other
         JAK2 rearrangements.

   - If receiving corticosteroids, must be a stable dose for >= 28 days prior to Day 1
   (unstable dosing not eligible).

   - Eastern Cooperative Oncology Group (ECOG) performance status =< 3.

   - Willing and able to review and execute informed consent (legally-authorized consent
   acceptable).

Exclusion Criteria:

   - Active life-threatening complication(s) from underlying eosinophilic disease (i.e.,
   leukostasis; acute thromboembolic disease including central nervous system (CNS)
   involvement; severe pulmonary or cardiac dysfunction). Stabilization of acute,
   life-threatening eosinophil-related co-morbidities will allow enrollment of the
   patient.

   - World Health Organization (WHO)-defined myeloid neoplasm associated with eosinophilia
   other than CEL NOS and JAK2 rearranged neoplasms (e.g., myelodysplastic syndrome
   (MDS); myeloproliferative neoplasms (MPN); MDS/MPN overlap disorders; and systemic
   mastocytosis (SM).

   - Reactive hypereosinophilia due to connective tissue disease, sarcoidosis or
   eosinophilic granulomatosis with polyangiitis.

   - Organ-restricted ?tissue? eosinophilia with the absence of peripheral eosinophilia in
   the blood.

   - Invasive malignancy over the previous 2 years except treated early stage carcinomas of
   the skin, completely resected intraepithelial carcinoma of the cervix, and completely
   resected papillary thyroid and follicular thyroid cancers.

   - Myeloid or lymphoid neoplasm with eosinophilia and abnormalities of PDGFRA, PDGFRB or
   FGFR1.

   - Anticipated to receive a hematopoietic stem cell transplant within the first 6 months
   of treatment on trial.

   - Major surgery within 4 weeks prior to entering the study.

   - Life expectancy of < 6 months.

   - Known diagnosis of human immunodeficiency virus (HIV).

   - Known diagnosis of chronic active hepatitis B or C (viral testing is not required).
   Subjects with a known history of hepatitis B and/or C are allowed on trial if at the
   time of enrollment, the virus is not active and undetected (testing required if there
   is a known history), and such patients are not actively receiving antiviral treatment
   specific for hepatitis B and/or C.

   - Clinically serious infections requiring ongoing antibiotic therapy.

   - Parasitic infection diagnosed within 24 weeks prior to enrollment.

   - Platelet count =< 25 x 10^9/L at baseline.

   - Alanine aminotransferase (ALT)/serum glutamate pyruvate transaminase (SGPT) > 4 x
   upper limit of normal (ULN) or direct bilirubin > 4 x ULN (if considered to be
   unrelated to the underlying eosinophilic disorder).

   - End-stage renal function (creatinine clearance [CrCl] < 15 mL/min or glomerular
   filtration rate [GFR] < 15 mL/min) regardless of whether hemodialysis is required.

   - Use of investigational or commercial therapies with the intent to treat the underlying
   eosinophilic disorder within 28 days of study start, including interferon; imatinib;
   alemtuzumab; cyclosporine; methotrexate; mepolizumab; benralizumab; or other antibody
   therapies.

   - Use of hydroxyurea within 7 days of study start.

   - Prior therapy with ruxolitinib or other JAK inhibitors.

   - Previous allergic reactions to JAK inhibitors or excipients.

   - Unwilling to commit to abstinence from heterosexual contact or agree to use and comply
   with highly effective contraception, 28 days prior to starting study drug, during the
   treatment period and for 12 weeks after discontinuation of study treatment.

   - Females of childbearing potential who have a positive pregnancy test (urine or serum)
   during screening period.