The discrepancy between self-reported smoking behavior and actual urine cotinine values among prenatal patients at four municipally operated clinical sites was examined. Face-to-face interview and birth certificate information about smoking behavior during pregnancy was compared with laboratory urine cotinine values for 74 patients. Almost three of every four (73%) self-reported nonsmokers had continine values greater than 80 ng/mL; one-half (48%) had values exceeding 100 ng/mL. Self-reported prenatal smoking behavior seems to be an unreliable indicator of actual smoking status among low-income prenatal patients, resulting in missed opportunities to lower tobacco-related exposure/risk among women with the poorest birth outcomes.