We describe a case of acute nonlymphocytic leukemia (ANLL) in a patient with a constitutional chromosome anomaly, inv(4)(p16q26). The patient had extensive occupational exposure to toxic chemicals. Reports of constitutional or acquired chromosome inversions in human malignancies are quite uncommon. The constitutional changes associated with hematologic malignancies include trisomy 21, balanced translocations, deletions, and sex chromosome anomalies. The breakpoints on chromosome 4 in our case are 4p16, to which the murine leukemia viral (v-raf) oncogene, pseudogene 1, has been mapped, and 4q26, which is the locus of the IL-2 gene. Activation of these genes could have played a role in the pathogenesis of the patient's leukemia.