Chromosomal analyses of myeloid cells were performed on ten patients who had acute nonlymphocytic leukemia (ANLL) following treatment for malignant lymphoma. Seven patients had Hodgkin disease and three had non-Hodgkin lymphoma, poorly differentiated lymphocytic type. Six patients were treated with radiotherapy and chemotherapy; two had radiotherapy only, and two chemotherapy only. The median time between diagnosis of lymphoma and subsequent leukemia was 58 mo. Four patients had the blast phase of a myeloproliferative syndrome, four had acute myelogenous leukemia, one had acute promyelocytic leukemia, and the tenth, erythroleukemia. None of four patients whose leukemia was treated with intensive chemotherapy responded. Every patient had an abnormal karyotype. Seven of the patients showed hypodiploid cell lines, two a pseudodiploid, and one a hyperdiploid cell line. Cells from every patient except one were lacking a B chromosome; in eight, this could be identified as a No. 5. Five of nine patients were lacking a No. 7. Loss or rearrangement of No. 17 was found in four and of Nos. 6 or 8 in three patients. Many of the karyotypes were bizarre, with marker chromosomes and minute chromosomes. The karyotypic pattern seen in these patients showed no correlation with the nature of the originalmore » lymphoma, the type of leukemia, or the therapy used. The chromosomal pattern of hypodiploid cell lines found in ANLL that arose de novo was similar to that occurring in treated lymphoma. However, in ANLL de novo, less than half of the patients had fewer than 46 chromosomes, and less than 10% had fewer than 45 chromosomes. In this study, 70% of the patients had fewer than 46 and 40% had fewer than 45 chromosomes. The critical question thus concerns the factors, as yet unknown, that predispose to the development of hypodiploid modal numbers in ANLL in lymphoma.« less