Our objective was to examine the efficacy of the added effect of individualized smoking relapse prevention counseling on obstetricians' and nurse midwives' usual advice during prenatal care. One hundred and seventy-five pregnant women who were smoking early in their pregnancy, but had quit by first prenatal visit, were randomly assigned to receive the usual advice from their obstetrician or nurse midwife, or usual advice plus individual relapse prevention counseling. Smoking status was measured by self-report, by urinary cotinine/creatinine ratio at the 36-week visit, and by self-report at long-term postpartum follow-up. We found that a smaller percentage of women in the intervention group (8.8%) reported smoking at the thirty-sixth-week visit than those in the usual care group (16.9%), a nonsignificant difference. No significant difference in relapse rates during pregnancy was observed based on urinary cotinine/creatinine ratios, but these rates, 29.5% and 27.9% respectively, were substantially higher than those based on self-report. The average number of days abstinent reported by women in the intervention group was significantly longer than that in the usual care group, 199 days versus 166 days respectively (P < .01). Logistic regression analysis indicated that longest time abstaining before first visit, level of belief in smoking's harm to the fetus, and motivation to smoke were independently related to the probability of relapsing to smoking by the 36-week visit. Long-term relapse rates were not significantly different: intervention group, 50.9%, usual care group, 50.0%.(ABSTRACT TRUNCATED AT 250 WORDS)