Late effects of treatment for Wilms' tumor. A report from the National Wilms' Tumor Study Group. The National Wilms' Tumor Study (NWTS) was initiated in 1969. One of its objectives was to modulate treatments according to risk factors to minimize the number and severity of treatment-related short-term and long-term iatrogenic complications. The NWTS has therefore incorporated a Long Term Follow-up Study (LTFS) within its framework to monitor late effects. The LTFS is confined to relapse-free survivors alive 5 years or longer after initial surgery, and data are collected using specifically designed forms. A total of 787 patients registered on NWTS-1 or NWTS-2 (1969 to 1979) were eligible, of whom 680 (86%) were available for analysis regarding musculoskeletal, cardiovascular, and neuropsychologic status, and the presence of benign and malignant tumors. Patients with early-stage disease who were treated with radiation had scoliosis reported, along with other musculoskeletal abnormalities (32 versus 2), nearly seven times as often as did the members of the cohort population who did not undergo radiation (35 of 57 versus 5 of 53, respectively). The difference in cardiovascular problems recorded in survivors who did and did not receive Adriamycin (Adria Laboratories, Columbus, OH) (2.4 versus 1.1 per 100-person years at risk) had borderline statistical significance (P = 0.06). No excess in neuropsychologic events was reported for those given the neurotoxin vincristine. When considering patients with disease of all stages, all 5 second malignant tumors occurred in the 623 patients who underwent radiation (RT patients); benign tumors were also more frequent in RT patients than in those patients who did not undergo radiation (41 of 486 or 8% versus 4 of 194 or 2%). Continuing study of this unique body of patients is needed, especially for those given Adriamycin, because of the known long interval needed for latent cardiomyopathy to become clinically manifest in some patients. 