The author studied urine cotinine excretion in 27 children who attended a research day care center to determine the extent of correlation between urine cotinine-creatinine ratios (CCR) and intensity of nicotine exposure in the home. Average nicotine levels in home air were determined by active air sampling during the evening and night hours on two consecutive days. Urine samples for cotinine and creatinine determinations were collected before, during, and after the two sampling periods. In addition, four sequential weekly urine samples for CCR were obtained from study children to determine the extent to which single determinations of CCR were representative for individual children. Fifteen children resided in homes with smokers and 12 did not. Urine CCR consistently distinguished most exposed and unexposed children. However, three exposed children had urine CCR's which clustered routinely around the borderline CCR (30 ng/mg cotinine-creatinine) used to distinguish exposed and unexposed children. In children exposed to environmental tobacco smoke in the home, there was a significant correlation between average home air nicotine levels and the logarighm of urine CCR the mornings following the home air monitoring periods (r = 0.57; p = 0.004). In study children, urine CCR's were remarkable stable over the one month observation period. Rank correlation coefficients for sequential weekly determinations of CCR were consistently greater than r = 0.89; p < 0.001.