Controversy exists regarding the most appropriate treatment for the rare adult patient who develops a so-called pediatric cancer. We have reviewed our 20-year experience with these patients and analyzed their outcome. A total of 299 patients with rhabdomyosarcoma (106), Wilms' tumor (97), and neuroblastoma (96) were evaluated and treated at Stanford University Medical Center between January 1967 and December 1987. Only 26 of these patients (8.7%) were diagnosed during "adulthood"; their age range was 18-67 years, median 23 years. Wilms' tumor; Five patients presented with Wilms' tumor at age greater than or equal to 18 years; four had unfavorable histology. All underwent multimodality therapy; however, only two have survived, one currently disease-free and one with disease. Neuroblastoma: Five patients presented with neuroblastoma at age greater than or equal to 18 years. Four underwent attempted surgical resection, post-operative irradiation (RT), and chemotherapy (CT); the other received no adjuvant CT. Only two of the five patients survive, both with disease. Rhabdomyosarcoma: Of the 16 adults (greater than or equal to 21 years) with rhabdomyosarcoma, 14 (87%) had advanced Intergroup Rhabdomyosarcoma Study-group disease (eight Group III, six Group IV). All 16 underwent aggressive multimodality therapy. At 10 months-16 years follow-up, only five patients survive, four of whom are apparently cured of their tumor. Neither histologic subtype nor site of presentation were of prognostic value. This series demonstrates that adults with Wilms' tumor, neuroblastoma, or rhabdomyosarcoma have a worse prognosis than do children with the same diagnosis. Possible explanations for this disparity in outcome include different tumor biology, less tolerance for treatment, and different natural history among adults relative to children.