Previous investigations of cancer survivors report that the cumulative incidence of subsequent leukemia plateaus between 10 and 15 years after primary therapy. Risk beyond 15 years has not been comprehensively assessed, primarily because of lack of long-term follow-up. Among 5-year survivors from the Childhood Cancer Survivor Study cohort, 13 pathologically confirmed cases of subsequent leukemia occurred ≥ 15 years after primary malignancy, with a mean latency of 21.6 years (range, 15-32 years). Seven were acute myeloid leukemia (2 acute promyelocytic leukemia with t(15;17), 2 with confirmed preceding myelodysplastic syndrome), 4 acute lymphoblastic leukemia (2 pre-B lineage, 1 T cell, 1 unknown), and 2 other. Two acute myeloid leukemia cases had the 7q- deletion. The standardized incidence ratio was 3.5 (95% confidence interval, 1.9-6.0). Median survival from diagnosis of subsequent leukemia was 2 years. This is the first description of a statistically significant increased risk of subsequent leukemia ≥ 15 years from primary diagnosis of childhood cancer.