Since more and more women in developed countries are delaying childbearing to an older age, it is important to find out whether birth defects, other than those resulting from chromosomal anomalies, are related to maternal age. We have studied all 26,859 children with birth defects of unknown aetiology identified among 576,815 consecutive livebirths in British Columbia. All these cases' records were linked with provincial birth records to allow determination of maternal age at birth. We excluded children with chromosomal anomalies and those with other birth defects of known aetiology. Only 3 of the 43 birth defect categories studied showed significant maternal-age-specific trends: there were decreasing linear trends with maternal age for patent ductus arteriosus (chi 2 = 36.65, 1 df, p less than 0.01) and hypertrophic pyloric stenosis (chi 2 = 4.90, 1 df, p less than 0.05) and a bell-shaped curve (risk increasing to maternal age 30 then falling) for congenital dislocatable hip/hip click. The findings from this population-based analysis of no association between the incidence of birth defects of unknown aetiology and advancing maternal age should be reassuring to healthy women who opt to delay childbearing.