• 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).