Recently, several specific chromosomal abnormalities have been associated with distinctive clinical and/or morphological subtypes of acute nonlymphocytic leukemia (ANLL). To further investigate the clinical utility of karyotype analysis in ANLL, we have examined G- banded metaphase chromosomes at diagnosis in 61 consecutive patients. Of the 60 patients who had adequate mitoses, 47 (78%) had a clonal chromosome abnormality. The sole karyotypic abnormality found in 5 patients was a del(16)(q22). The unique pretreatment characteristic of these 5 patients was marrow eosinophilia ranging from 8% to 54%. No other patient had more than 4% marrow eosinophils. Among the patients with eosinophilia, all had Auer rods, serum muramidase was elevated in the 4 tested, and 4 had hepatomegaly at presentation. Both patients who survived initial treatment remain in complete remission at 23+ and 33+ mo. The data suggest that we have identified a new cytogenetic-clinical subtype of ANLL defined by the del(16)(q22)