Nausea and vomiting during pregnancy (NVP) has been associated with favorable pregnancy outcome, though little is known about factors influencing its occurrence. In this study, information on NVP in 210 patients at a west coast health maintenance organization was obtained. Smoking and alcohol consumption before and during pregnancy were also estimated in two personal interviews during gestation. In all, 72% of the subjects had NVP in the first 4 months of pregnancy. Smokers had significantly less NVP than non-smokers (52% vs. 79%). Furthermore, NVP in smokers was negatively associated with alcohol consumption before and during pregnancy, with the stronger relation being for alcohol reported in the 6 months before pregnancy; only 46% of smokers drinking more than 1/2 fl. oz. of absolute alcohol daily in this period reported NVP, while 68% of smokers drinking less had NVP. For non-smokers, there was no relation between alcohol use in any period and NVP. The risk of NVP associated with pre-pregnancy drinking was not related to any change in alcohol consumption after conception. These results suggest an interaction between NVP, smoking, and reported alcohol consumption. The association of favorable pregnancy outcome with NVP may be in part a reflection of moderation in maternal alcohol and tobacco use.