Relations between birth characteristics and risk of primary brain tumor were explored in a prospective study of the 1,489,297 children born in Norway between 1967 and 1992. A total of 459 primary brain tumors, including 78 medulloblastomas and 168 astrocytomas, were diagnosed in the age interval 0-15 years. The overall risk of brain tumor depended on the season of birth (p = 0.01), with a higher risk for children born in winter than those born in spring [incidence rate ratio (IRR) = 1.52; 95% confidence interval (CI) 1.18-1.97]. An inverse association was observed with father's age at birth of child. The risk of medulloblastoma was positively associated with birth weight (IRR = 1.27/500 g; p = 0.05). Inverse relationships with length at birth were found for astrocytoma in the 0-1 and 5-10 year age intervals. Among 5-10-year-old children, birth weight was also inversely related to risk of astrocytoma. Our results suggest that risk factors may differ over age intervals and histological subgroups.