Passive smoking has been shown to adversely affect the health of infants and children. Black children and adults appear to be more susceptible to a variety of tobacco smoke health hazards for unknown reason. The objectives of this study were as follows: (1) to correlate the number of cigarettes reported to have been smoked by parents with urine and hair concentrations of cotinine in children; and (2) to identify race differences in systemic exposure to cotinine in children. This was an observational study in a consulting pediatric office on 169 nonsmoking children between 2 and 18 years of age, not actively smoking. The outcome measures of interest were urinary cotinine concentrations corrected for milligram of creatinine and hair concentration of cotinine (per milligram of hair). There were significant correlations between the number of cigarettes the child was exposed to and urinary cotinine (r = 0.68, p = 0.0001) or hair cotinine concentrations (r = 0.19, p = 0.02), and between urinary and hair cotinine (r = 0.3, p = 0.0005). In this cohort, parents of black children (n = 21) tended to smoke less (6.6 +/- 3/d, mean +/- SEM) than white parents (n = 97) (12 +/- 1.8, mean +/- SEM) (p = 0.2). Despite being exposed to less cigarettes, black children had higher hair concentrations of cotinine than white children (0.89 +/- 0.25 ng/mg vs 0.48 +/- 0.05 ng/mg; p = 0.05). The ratio hair/urine concentrations of cotinine was twofold higher in black children (0.035 +/- 0.01 vs 0.019 +/- 0.002; p = 0.004). White children with dark hair did not differ significantly from white children with fair hair in any of these indexes. The amount of urinary cotinine per milligram of creatinine caused by 1 cigarette per day was twofold higher in black children (14.7 +/- 5.2 ng/mg of creatinine) than in white children (6.3 +/- 1.2 ng/mg of creatinine) (p = 0.02). These data suggest that black children handle cigarette smoke differently from white children and that black children have higher systemic exposure to this constituent of cigarette smoke.