To assess the proportion of children with cancer who have been managed by mainstream pediatric cancer programs, population-based cancer incidence data for Los Angeles County (LAC) children (under 20 years of age) for the years 1972 through 1987 were linked with patient records of children registered with the two national cooperative pediatric oncology groups, Children's Cancer Study Group and Pediatric Oncology Group. The proportion of children with cancer who were registered by cooperative groups increased markedly over time: 9% of LAC children younger than 15 years of age who were diagnosed with cancer in 1972 were registered with cooperative groups, compared to 52% of those diagnosed in 1980 and 62% of those diagnosed in 1987. Registration rates decreased with increasing age at cancer diagnosis. In the most recent time period, 1984-1987, 66% of LAC children diagnosed with cancer under age 5 years were registered with cooperative groups compared to 62% of those who were 5 to 9 years old and 49% of those who were 10 to 14 years old; although they were frequently diagnosed with tumors considered to be childhood cancers, only 19% of older adolescents (aged 15-19 years) were registered. In LAC, there was no apparent bias in registration rates with regard to gender or racial-ethnic background. Among patients diagnosed in the period 1984-1987, children in the highest of five socioeconomic status categories were underrepresented among registrants. Registration rates were highest (70% or greater) for patients with acute lymphocytic and acute nonlymphocytic leukemia, medulloblastoma, hepatoblastoma, Wilms tumor, and rhabdomyosarcoma. Fewer than 50% of patients with other brain and central nervous system tumors, retinoblastoma, other soft tissue sarcomas, and bone tumors were registered with the cooperative groups.