Previous studies have found a summer peak in the birth of individuals with Down's syndrome but have not tended to examine them into adulthood. The aims of this study were to look for evidence of a seasonal effect in birth and to uncover any differences in adulthood between those born in different seasons. The casenotes of all adults with Down's syndrome from the catchment area of a hospital for people with learning disability were examined. A summer peak in births was confirmed. Only 6% of births took place during January and February compared with the 17% expected (P = 0.019); birth during these 2 months was associated with female sex (P = 0.047). There was a trend for those born in December to March not to develop epilepsy (P = 0.053).