An analysis of R-banded PHA-stimulated lymphocytes from 13 patients with secondary acute non-lymphocytic leukemia (S-ANLL) following breast cancer or lymphoma, and treatment by alkylating agents and/or radiotherapy, is reported. We found that chromosomes 5, 7, 11 and 17 are over-involved in structural rearrangements. These anomalies are similar to those observed in the same categories of patients without S-ANLL, and after in vitro treatment of normal lymphocytes by the alkylating agent melphalan. These anomalies are thus likely to be induced by treatment, independently of S-ANLL. However, the same chromosomes (5, 7, 11 and 17) are recurrently deficient in leukemic S-ANLL clones. In spite of these similarities, it remains unlikely that the deficiencies observed in leukemic clones were directly induced at the time of treatment. Probably, treatment of primary cancers induces nonrandom mutations of recessive genes located on these chromosomes as also indicated by chromosomal lesions. Various rearrangements including deletions of the homologous normal counterparts may then occur, unmasking mutated recessive genes. The latter stage would be concomitant with the leukemogenic process.