This study consists of 65 patients (pts) who developed a myelodysplastic syndrome (MDS) (39 pts) or acute myeloid leukemia (AML) (26 pts) following chemotherapy and/or radiotherapy; the interval from the onset of therapy to bone marrow abnormality ranged from 11 to 192 months (median, 58). Thirty-three patients had been previously treated for lymphoproliferative diseases, 29 for carcinoma, and three for a nonneoplastic disorder. Approximately 30% of the cases presenting in the MDS phase evolved to AML in one to 12 months (median, 3.5). The AML in 49% of the cases was not readily classified according to French- American-British (FAB) criteria; the primary difficulty in classification related to the involvement of multiple cell lines. Among the cases that could be classified, all FAB types were represented except for M1; M2 was the most frequent type. Clonal chromosome abnormalities were found in marrow specimens from 22 of 24 (92%) patients studied with G banding; 11 had abnormalities of chromosomes 5 and/or 7. The median survival for all patients was four months with no significant difference between those treated and not treated with antileukemic therapy. The median survival was three months for the patients presenting with AML, six months for the patients with AML following an MDS, and four months for the patients with an MDS that did not evolve to AML. The findings in the present study suggest that there are three stages of therapy-related panmyelosis: (1) pancytopenia with associated myelodysplastic changes, (2) a frank MDS, and (3) overt AML. Many patients will present in the stage of overt AML that differs from de novo AML primarily by the high incidence of trilineage involvement, difficulty in classification, frequent cytogenetic abnormalities, and poor response to antileukemic therapy. The myelodysplastic phase, with or without evolution to acute leukemia, is a highly lethal disease with a median survival comparable to that of the patients who present with AML.