Objectives: To examine the association between use of a dummy (pacifier) durin g sleep and the risk of sudden infant death syndrome (SIDS) in relation to other risk factors. Design: Population based case-control study. Setting: Eleven cou nties in California. Participants: Mothers or carers of 185 infants whose deaths were attributed to SIDS and 312 randomly selected controls matched for race or ethnicity and age. Main outcome measure: Use of a dummy during sleep determined through interviews. Results: The adjusted odds ratio for SIDS associated with us ing a dummy during the last sleep was 0.08 (95%confidence interval 0.03 to 0.21 ). Use was associated with a reduction in risk in every category of sociodemogra phic characteristics and risk factors examined. The reduced risk associated with us e seemed to be greater with adverse sleep conditions (such as sleeping prone or on side and sleeping with a mother who smoked), although the observed interactio ns were not significant. For example, infants who did not use a dummy and slept prone or on their sides (v on their back) had an increased risk of SIDS (2.61, 1 .56 to 4.38). In infants who used dummies, there was no increased risk associate d with sleeping position (0.66, 0.12 to 3.59). While cosleeping with a mother wh o smoked was also associated with increased risk of SIDS among infants who did n ot use a dummy (4.5, 1.3 to 15.1), there was no such association among those who did (1.1, 0.1 to 13.4). Conclusions: Use of a dummy seems to reduce the risk of SIDS and possibly reduces the influence of known risk factors in the sleep envi ronment. Objectives: To examine the association between use of a dummy (pacifier) ​​durin g sleep and the risk of sudden infant death syndrome (SIDS) in relation to other risk factors. California. Participants: Mothers or carers of 185 infants whose deaths were attributed to SIDS and 312 randomly selected controls matched for race or ethnicity and age. Main outcome measures: Use of a dummy during sleep determined through interviews. Results: The adjusted odds ratio for SIDS associated with us a dummy during the last sleep was 0.08 (95% confidence interval 0.03 to 0.21). Use was associated with a reduction in risk in every category of sociodemogra phic characteristics and risk factors examined. seemed to be greater with adverse sleep conditions (such as sleeping prone or on side and sleeping with a mother who smoked), although the observed interactio ns were not significant. For examp le, infants who did not use a dummy and slept prone or on their sides (v on their back) had an increased risk of SIDS (2.61, 1.56 to 4.38). In infants who used dummies, there was no increased risk associate d with sleeping position (0.66, 0.12 to 3.59). While cosleeping with a mother wh o smoked was also associated with increased risk of SIDS among infants who did n ot use a dummy (4.5, 1.3 to 15.1), there was no such association Among those who did (1.1, 0.1 to 13.4). Conclusions: Use of a dummy seems to reduce the risk of SIDS and possibly reduces the influence of known risk factors in the sleep envi ronment.