Eleven therapy related leukemias (TRL) who were hospitalized in the Department of Hematology and Chemotherapy, Kanagawa Cancer Center between October 1983 and December 1993 were identified. Six of the patients were males and five were females. Their median age was 62 years (range from 14 to 75). Three patients had previously received treatment for breast cancer and two patients for malignant lymphoma. The other patients had received treatment for lung cancer, urinary bladder cancer, gastric cancer, brain tumor, maxillary sinus cancer and macroglobulinemia, respectively. Seven patients had been treated with chemotherapy and four patients had been treated with chemotherapy and irradiation for the primary tumor. The TRL cases consisted of 8 acute non-lymphoid leukemias, two acute lymphoid leukemias and one hypoplastic leukemia, respectively. The status of primary tumors at the development of TRL was complete remission in ten patients and partial remission in one patient. Three of the 10 patients who received anti-leukemic therapy entered complete remission and the median survival time was 36 days (from 7 days to 489 days). One patient expired of pneumonia before he received anti-leukemic therapy. TRL patients showed poor response to chemotherapy and had poor prognosis. These data suggest that the use of reduced doses of carcinogenic drugs for primary tumors might be required to prevent the development of TRL.