During the last decade several studies have shown that children whose parents smoke have higher rates of asthma. Recently, hair concentrations of cotinine have been shown to reflect systemic exposure to this constituent of smoke in both children and adults. At the present time it is not known, however, why some children exposed to passive smoking have asthma while others, similarly exposed, do not. The present study aimed at verifying whether asthmatic children are different from nonasthmatic children exposed to similar degrees of passive smoking in the way their bodies handle nicotine, a constituent of cigarette smoke. Seventy-eight asthmatic children were compared to 86 control children, all attending a consulting pediatric clinic in Toronto. A questionnaire completed by the parents and children detailed the daily number of cigarettes the child was exposed to and the identity of the smokers. Clinical data were extracted from the patients' charts. Urinary (corrected for creatinine) and hair concentrations of cotinine were measured by radioimmunoassays. The asthmatic and control children were of similar age, gender, and ethnic distribution, parental education, and socioeconomic status. Parents of asthmatic children tended to report a lower daily number of cigarettes (7.4 +/- 1.3/day vs. 11.2 +/- 2.3/day, p = 0.14), and this report agreed with the trend of urinary cotinine (47.1 +/- 9.1 ng/mg vs. 62.6 +/- 11.5 ng/mg, respectively). Conversely, children with asthma had on average twofold higher concentrations of cotinine in their hair (0.696 +/- 0.742 ng/mg) than control children (0.386 +/- 0.383) (p = 0.0001). In a similar manner, the hair:urine concentration ratio was significantly higher in children with asthma (0.028 +/- 0.002) than in their controls (0.18 +/- 0.003) (p = 0.0001). These results suggest that under exposure to similar amounts of nicotine, children with asthma have on average twofold higher systemic exposure to this constituent of cigarette smoke. These data suggest that out of all children passively exposed to environmental tobacco smoke, those who exhibit asthma have a higher systemic exposure to nicotine, possibly due to lower clearance rate. This is the first evidence of pharmacokinetic predisposition to environmental tobacco smoke as an etiological factor in pediatric asthma.