We describe a patient with eosinophilia and an abnormal CD3+4−8−αβ+ T‐cell population. Chromosomal analysis of sorted CD3+4−8− cells revealed abnormal karyotypes on chromosome 16. In the presence of IL‐2 the production of IL‐5 from CD3+4−8− cells was higher than that from CD3+4+/8+ cells. Eosinophil survival‐enhancing activity in the patient serum was inhibited by a combination of anti‐IL‐5 and anti‐GM‐CSF monoclonal antibodies. These data suggest that increased production of IL‐5 and GM‐CSF from the abnormal CD3+4−8− cells might cause eosinophilia.