We studied the risk of the development of acute myeloid leukemia (AML) during initial remission in 733 consecutive children with acute lymphoid leukemia (ALL) who were treated with intensive chemotherapy. This complication was identified according to standard morphologic and cytochemical criteria in 13 patients 1.2 to 6 years (median, 3.0) after the diagnosis of ALL. At three years of follow-up, the cumulative risk of secondary AML during the first bone marrow remission was 1.6 percent (95 percent confidence limits, 0.7 and 3.5 percent); at six years, it was 4.7 percent (2 and 10 percent). The development of secondary AML was much more likely among patients with a T-cell than a non-T-cell immunophenotype (cumulative risk, 19.1 percent [6 and 47 percent] at six years). Sequential cytogenetic studies in 10 patients revealed entirely different karyotypes in 9, suggesting the induction of a second neoplasm. In eight of these patients, the blast cells had abnormalities of the 11q23 chromosomal region, which has been associated with malignant transformation of a pluripotential stem cell. There was no evidence of loss of DNA from chromosome 5 or 7, a karyotypic change commonly observed in cases of AML secondary to treatment with alkylating agents, irradiation, or both. We conclude that there is a substantial risk of AML in patients who receive intensive treatment for ALL, especially in those with a T-cell immunophenotype, and that 11q23 chromosomal abnormalities may be important in the pathogenesis of this complication.