Over a 6-year period, 275 patients were treated with autologous bone marrow transplantation (auto-BMT) for advanced-stage malignant lymphoma. After BMT, clonal chromosomal abnormalities were detected in hematopoietic cells from 10 patients. All 10 had morphologically and cytogenetically normal BMs at the time of stem cell harvest. The cytogenetic changes were first detected 1.8 to 6.5 years (mean, 3.9) after induction chemotherapy, and 0.5 to 3.1 years (mean, 1.4) after transplantation, and were characteristic of those reported for therapy- related myelodysplastic syndrome (MDS) in 9 of the patients: abnormalities of chromosome 5 or 7 (classical-form) were present in 4, 11q23 or 21q22 abnormalities (topoisomerase II-related form) were detected in 3, and a combination of both forms was seen in 2 patients. Clonal 2p abnormalities were found in the 1 remaining patient. The abnormal karyotypes were associated with morphologically recognizable MDS in 3 patients and with acute myeloid leukemia (AML) arising in MDS in 2. Four of these patients have died: 3 of AML and 1 of infection. One patient is still alive with cytopenia. The clonal cytogenetic abnormalities were not associated with MDS in 5 patients: 1 has died of recurrent lymphoma, 2 have cytopenia, and 2 still have no morphologic or clinical evidence of MDS after short follow-up (4 and 13 months). Compared with a control group matched for disease, length of follow-up, and treatment with auto-BMT, there were no statistically significant associations between the development of clonal chromosomal abnormalities and age, number of chemotherapeutic regimens, prior local radiation, BMT conditioning regimen (with or without total body irradiation), or type of lymphoma. These studies show that the risk of developing clonal cytogenetic changes after auto-BMT for malignant lymphoma is approximately 9% at 3 years, even when pre-BMT karyotypic studies are normal. The exact significance of these cytogenetic abnormalities in the absence of MDS or AML is unclear.