Aims: To determine the combined effects of sudden infant death syndrome (SIDS) risk factors in the sleeping environment for infants who were “ small at birth” (pre-term (< 37 weeks), low birth weight (< 2500 g), or both). Methods: A three year population based, case-control study in five former health regions in England (population 17.7 million) with 325 cases and 1300 controls. Parental interviews were carried out after each death and reference sleep of age matched controls. Results: Of the SIDS infants, 26% were “ small at birth” compared to 8% of the controls. The most common sleeping position was supine, for both controls (69% ) and those SIDS infants (48% ) born at term or ≥ 2500 g, but for “ small at birth” SIDS infants the commonest sleeping position was side (48% ). The combined effect of the risk associated with being “ small at birth” and factors in the infant sleeping environment remained multiplicative despite controlling for possible confounding in the multivariate model. This effect was more than multiplicative for those infants placed to sleep on their side or who shared the bed with parents who habitually smoked, while for those “ small at birth” SIDS who slept in a room separate from the parents, the large combined effect showed evidence of a significant interaction. No excess risk was identified from bed sharing with non-smoking parents for infants born at term or birth weight ≥ 2500 g. Conclusion: The combined effects of SIDS risk factors in the sleeping environment and being pre-term or low birth weight generate high risks for these infants. Their longer postnatal stay allows an opportunity to target parents and staff with risk reduction messages. Aims: To determine the combined effects of sudden infant death syndrome (SIDS) risk factors in the sleeping environment for infants who were “small at birth” (pre-term (<37 weeks), low birth weight (<2500 g), or both), three-year population based, case-control study in five former health regions in England (population 17.7 million) with 325 cases and 1300 controls. Parental interviews were carried out after each death and reference sleep of age matched controls. Results: Of the SIDS infants, 26% were “small at birth” compared to 8% of the controls. The most common sleeping position was supine, for both controls (69%) and those SIDS infants (48%) born at term or ≥ 2500 g, but for “small at birth” SIDS infants the commonest sleeping position was side (48%). The combined effect of the risk associated with being “small at birth” and factors in the infant sleeping environment remained multiplicativeghanough controlling for possible confounding in the mu This effect was more than multiplicative for those infants placed a sleep on their side or who shared the bed with parents who habitually smoked, while for those “small at birth” SIDS who slept in a room separate from the parents, the large combined effect showed evidence of a significant interaction. No excess risk was identified from bed sharing with non-smoking parents for infants born at term or birth weight ≥ 2500 g. Conclusion: The combined effects of SIDS risk factors in the sleeping environment and being pre -term or low birth weight generate high risks for these infants. Their longer postnatal stay allows an opportunity to target parents and staff with risk reduction messages.