Well‐known epidemiological features of sudden infant death syndrome (SIDS) are age at death and the increased numbers in winter. There are more SIDS deaths in late autumn/early winter and there is a seasonal rhythm of births with a peak in late summer and early autumn. The data set was 14 033 SIDS deaths from Scotland, England and Wales over the 11 years 1982–92. Using log‐linear models, which accounted for age at death and month of death, birth month was found to be a statistically significant risk factor for SIDS independent of age at death and winter environment (P<0.001). Although winter season had the largest effect (relative risk 2.7 in January compared with August), the independent effect of birth month was of clinical as well as statistical significance with a relative risk for August births of 1.37 compared with those born in April. The analysis of each birth month cohort revealed a change in age distribution with infants born in early winter (December) dying at a younger age (mean 108 days) than those born in midsummer (June) (mean 146 days). Although winter season and age are the most influential factors, the substantial effect of month of birth requires explanation and points to as yet unidentified environmental influences during pregnancy.