Background To examine the social determinants of infant smoke exposure and whether these associations vary by mothers' smoking status and number of children in the household. Methods There were 135 278 mothers from 28 states who participated in the Pregnancy Risk Assessment Monitoring System from 2000 to 2003. Results Overall 9.9% of mothers reported that her infant was in the same room as someone smoking for 1 or more hours per day. Among smoking households, infants with 1 or 2+ siblings were 25% and 59% more likely to be exposed to 1+ hours of second‐hand smoke daily, respectively, than infants with no siblings. Conclusion Infants only a few months old are being exposed to second‐hand smoke, particularly infants whose mother smokes and infants with siblings, indicating the importance of surveillance even for this young age group.