OBJECTIVE: To investigate the relation between routine infant care practices and the sudden infant death syndrome in Scotland. METHODS: National study of 201 infants dying of the sudden infant death syndrome (cases) and 276 controls by means of home interviews comparing methods of infant care and socioeconomic factors. RESULTS: Sleeping prone (odds ratio 6.96 (95% confidence interval 1.51 to 31.97) and drug treatment in the previous week (odds ratio 2.33 (1.10 to 4.94)) were more common in the cases than controls on multivariate analysis. Smoking was confirmed as a significant risk factor (odds ratio for mother and father both smoking 5.19 (2.26 to 11.91)). The risk increased with the number of parents smoking (P < 0.0001), with the number of cigarettes smoked by mother or father (P = 0.0001), and with bed sharing (P < 0.005). A new finding was an increased risk of dying of the syndrome for infants who slept at night on a mattress previously used by another infant or adult (odds ratio 2.51 (1.39 to 4.52)). However, this increased risk was not established for mattresses totally covered by polyvinyl chloride. CONCLUSIONS: Sleeping prone and parental smoking are confirmed as modifiable risk factors for the sudden infant death syndrome. Sleeping on an old mattress may be important but needs confirmation before recommendations can be made.