Forty of the 550 patients (7%) entered to the 11q23 Workshop had secondary (s) acute lymphoblastic leukemia (nine cases), s-acute myeloid leukemia (25 cases, predominantly of FAB type M5), s-acute leukemia unspecified (one case) or s-myelodysplastic syndrome (five cases) following treatment for a primary malignancy. Breast cancer (12 cases) and non-Hodgkin's lymphoma (eight cases) were the most frequent primaries. Twenty-three patients had been treated with either an epipodophyllotoxin (seven patients) or an anthracycline (10 cases) or both (four cases) frequently combined with alkylating agents (12 cases) and with radiotherapy (six cases). Two further patients had alkylating agents and two had radiotherapy alone. Time between diagnosis of the primary and secondary malignancy was between 10 months and 22 years (median 24 months). The incidence of secondary malignancies in 11q23 subgroups was: t(11;19)(q23;p13.1) (33%), t(9;11) (8%), t(4;11) (5.5%), t(10;11)(5%), t (6;11) (3%), del11q23(2%) and 10 patients had a rare 'other' abnormality. No associations were seen between type of prior malignancy and 11q23 subgroup, or between prior malignancy and leukemia subtype. Remission, when achieved (32 patients), was short (median 5 months). Two patients survived following a bone marrow transplant for s-leukemia and s-myelodysplastic syndrome.