To document passive smoke exposure, we measured concentrations of serum cotinine, a major metabolite of nicotine, in 38 young children and compared the results with the smoking histories of home residents. Cotinine was detected in 26 children (68%), of which ten had no household exposure according to a questionnaire. The serum cotinine concentration was significantly elevated in blacks compared with whites after controlling for the number of smokers in the home. After stratifying by race, there was a significant direct correlation between the serum cotinine concentration and the number of smokers in the home, the amount smoked by the mother, and the amount smoked by others in the home. We conclude that the serum cotinine concentration is a useful indicator of the actual exposure of young children to tobacco smoke and that unexplained racial differences in cotinine levels exist.