Several adverse birth outcomes are associated with cigarette smoking. It is important to determine the prevalence of cigarette smoking among pregnant low-income women and to evaluate their smoking cessation patterns in order to target appropriate interventions. Ethnically diverse pregnant women aged 15-45 years were recruited from Minneapolis or Saint Paul Women, Infants, and Children (WIC) clinics before their third trimester. Serum cotinine levels were assayed for 98 women and compared with self-report. The women were unaware that their smoking status would be validated. Twenty-one (21%) women had a positive serum cotinine value (> or =3 ng/mL); 16 (76%) admitted smoking within the previous 24 h before interview and five denied smoking. Of the five, four had cotinine levels that could suggest passive smoke exposure. Thirty-seven women (38%) admitted cigarette smoking during the pregnancy but before knowing that they were pregnant; 18 (49%) of these denied current smoking at the interview and also presented with negative cotinine levels. These data suggest that some participants in WIC make a concerted effort to quit smoking when they find out they are pregnant, and are generally truthful when reporting their smoking habits during pregnancy.