Environmental factors operating around the time of birth may influence the subsequent development of childhood cancer, particularly leukaemia. Certain factors may vary with season (e.g. infections), and we therefore investigated whether there was any evidence of seasonality of month of birth, based on data from 4199 children diagnosed with cancer under the age of 15 years. We extracted details of (i) children born 1960-95 and diagnosed 1968-95 from two population-based registries, covering the Northern (n = 2053) and Yorkshire (n = 1977) Regional Health Authority and (ii) children born in Cumbria 1950-93 and diagnosed anywhere in the UK before 1994 from a birth cohort database (n = 397). The following diagnostic categories were analysed: 0-14 years--all cancers, leukaemias, acute lymphoblastic leukaemias, central nervous system tumours, all other solid tumours; 1-6 years--leukaemias, acute lymphoblastic leukaemias. Seasonal variation was tested using Walter and Elwood's test, and logistic regression analysis allowing for cyclical variation in month of birth. No evidence of seasonality was present for any group except acute lymphoblastic leukaemia diagnosed among 1- to 6-year-olds. Seasonal trends varied by region: in the Northern and Cumbrian datasets, seasonality patterns were significant and similar (P < 0.05) with a predicted peak in early spring, whereas in Yorkshire there was less strong evidence of seasonality (P = 0.08) with a peak predicted in late summer. These findings suggest that local seasonal environmental factors operating around the time of birth are not associated with the totality of childhood cancer, but possible links with acute lymphoblastic leukaemia are supportive of a hypothesis of an infectious aetiology.