Objective: To determine whether seasonality of the sudden infant death syndrome persists now that rates have fallen, mostly after widespread adoption of the “face upwards” sleeping position. Design: Monthly data on the sudden infant death syndrome during 1987-9 were compared for seasonality with those of 1991-3; rates were studied as deaths per 1000 live births. Setting: Australia and Britain (England, Wales, and Scotland). Subjects: Infants under 1 year dying of the syndrome (2401 for Australia and 6630 for Britain). Main outcome measure: Extent of seasonal variation (amplitude) was established by cosinor analysis; amplitudes for the earlier and later years were compared. Results: The rate fell in every month, and, though it did so relatively more in winter than summer, seasonality remained a distinctive feature. In the comparison of amplitudes the ratio between the earlier and later years was 1.4 in both Australia and Britain. Some differences between the hemispheres were noted. Conclusions: Seasonality of the sudden infant death syndrome remains to be explained and continues to be an important aetiological lead. Studies from other countries are needed. Analysis of two large series has shown that seasonal variation in incidence persists but has been reduced by about one quarter Seasonal variation is an unexplained epidemiological feature with the potential to be an important clue to aetiology