BACKGROUND: Childhood cancer survivors develop gastrointestinal cancer more frequently and at a younger age than the general population, but the risk factors have not been well-characterized. OBJECTIVE: To determine the risk and associated risk factors for gastrointestinal subsequent malignant neoplasms (SMNs) in childhood cancer survivors. DESIGN: Retrospective cohort study. SETTING: The Childhood Cancer Survivor Study, a multicenter study of childhood cancer survivors diagnosed between 1970 and 1986. PATIENTS: 14 358 survivors of cancer diagnosed when they were younger than 21 years of age who survived for 5 or more years after the initial diagnosis. MEASUREMENTS: Standardized incidence ratios (SIRs) for gastrointestinal SMNs were calculated by using age-specific population data. Multivariate Cox regression models identified associations between risk factors and gastrointestinal SMN development. RESULTS: At median follow-up of 22.8 years (range, 5.5 to 30.2 years), 45 cases of gastrointestinal cancer were identified. The risk for gastrointestinal SMNs was 4.6-fold higher in childhood cancer survivors than in the general population (95% CI, 3.4 to 6.1). The SIR for colorectal cancer was 4.2 (CI, 2.8 to 6.3). The highest risk for gastrointestinal SMNs was associated with abdominal radiation (SIR, 11.2 [CI, 7.6 to 16.4]). However, survivors not exposed to radiation had a significantly increased risk (SIR, 2.4 [CI, 1.4 to 3.9]). In addition to abdominal radiation, high-dose procarbazine (relative risk, 3.2 [CI, 1.1 to 9.4]) and platinum drugs (relative risk, 7.6 [CI, 2.3 to 25.5]) independently increased the risk for gastrointestinal SMNs. LIMITATION: This cohort has not yet attained an age at which risk for gastrointestinal cancer is greatest. CONCLUSION: Childhood cancer survivors, particularly those exposed to abdominal radiation, are at increased risk for gastrointestinal SMNs. These findings suggest that surveillance of at-risk childhood cancer survivors should begin at a younger age than that recommended for the general population. PRIMARY FUNDING SOURCE: National Cancer Institute.