A few previous studies have revealed an increased risk for sudden infant death syndrome (SIDS) during weekends and holidays suggesting environmental factors as potential trigger mechanisms for death. In the present study, the weekend/holiday effect has been assessed on the basis of 1480 SIDS cases in 19 Norwegian counties during a 19-year period (1967-85). The phenomenon has been confirmed for 17 of the 19 counties. Special attention has been attached to differences in the geographical and seasonal distribution of the phenomenon. Deaths of other causes in infancy did not display such a dependency on the day of the week.