The smoking habits of 56 families who lost babies to the sudden infant death syndrome (SIDS) were compared to those of 86 control families. A higher proportion of SIDS mothers smoked both during pregnancy (61 vs. 42%) and after their babies were born (59 vs. 37%). SIDS mothers also smoked a significantly greater number of cigarettes than controls. Exposure to cigarette smoke (passive smoking) appears to enhance the risk of SIDS for reasons not known.