We examined the prevalence and level of environmental tobacco smoke (ETS) exposure using urine cotinine levels in children and three types of maternal self-reports in a sample of 196 low socioeconomic status (SES) mothers and their children. The self-reports were report of the mother's own smoking, report of the number of household smokers, and report of the usual number of daily hours that the child was exposed to ETS. According to the reports, 59% of the children's mothers were current smokers, 71% of the children came from households with smokers, and 85% had daily exposure to ETS. Based on urine cotinine measures adjusted for creatinine, 79% of the children were identified as ETS exposed. The average urine cotinine level was 19.6 ng/mg, and the median cotinine level was 13.1 ng/mg (range: 0-120 ng/mg). Correlations between urine cotinine levels and the three maternal reports were 0.48, 0.43, and 0.36, respectively. The most sensitive maternal report measure was number of hours of exposure per day when using adjusted urine cotinine levels of >/=5 ng/mg were used as the yardstick of exposure. Of the 154 children screened at the level of >/=5 ng/mg, 89.5% were also identified by maternal report as ETS exposed. However, 30 children whose mothers reported ETS exposure had urine cotinine levels of <5 ng/mg. These data showed that ETS exposure was prevalent in low-SES children and that the maternal reports identified a higher number of children as ETS exposed. The biological measures provided data on levels of recent exposure; however, level of exposure from biological measures correlated only moderately with the maternal report. A combination of a maternal report and a biological measure is suggested as the most informative estimate of ETS exposure in young children.