GH is increasingly used for treatment of children and adults. It is mitogenic, however, and there is therefore concern about its safety, especially when used to treat cancer patients who have become GH deficient after cranial radiotherapy. We followed 180 children with brain tumors attending three large hospitals in the United Kingdom and treated with GH during 1965-1996, and 891 children with brain tumors at these hospitals who received radiotherapy but not GH. Thirty-five first recurrences occurred in the GH-treated children and 434 in the untreated children. The relative risk of first recurrence in GH-treated compared with untreated patients, adjusted for potentially confounding prognostic variables, was decreased (0. 6; 95% confidence interval, 0.4-0.9) as was the relative risk of mortality (0.5; 95% confidence interval, 0.3-0.8). There was no significant trend in relative risk of recurrence with cumulative time for which GH treatment had been given or with time elapsed since this treatment started. The relative risk of mortality increased significantly with time since first GH treatment. The results, based on much larger numbers than previous studies, suggest that GH does not increase the risk of recurrence of childhood brain tumors, although the rising trend in mortality relative risks with longer follow-up indicates the need for continued surveillance.