The present study was conducted to examine several different methods and cutpoints for determining smoking status in pregnant and recently postpartum women. Self-reported smoking status, urine cotinine levels determined by gas chromatography (GC) and by enzyme immunoassay testing (EMIT), and breath carbon monoxide (CO) levels were assessed at 28 weeks antepartum and 12 and 24 weeks postpartum in 131 women enrolled in studies on smoking cessation and relapse prevention. Classifications based on urine-cotinine GC testing served as the standard in most analyses. Overall agreement between self-reported smoking status and classification based on urine-cotinine GC testing was excellent (> or =95%) at several cutpoints (50, 25, and 12.5 ng/ml) but highest at 25 ng/ml. Classifications based on EMIT urine cotinine levels were in nearly perfect (> or =98%) agreement with those made by GC when the cutpoint for the former was set at approximately 80 ng/ml (79-87 ng/ml). Classifications based on breath CO were in relatively poor agreement (< or =87%) with GC classifications at all cutpoints examined but best at 4 ppm. Overall, these results provide detailed information on several commonly used methods for classifying smoking in pregnant and recently postpartum women that should be practically useful to researchers and clinicians involved in efforts to eliminate smoking in this population.