The aim of this study was to investigate the effect of alcohol and caffeine consumption on birth weight and the possible interaction of these substances with smoking. The sample included 628 women who were interviewed at their first visit to the maternity hospital of Roubaix, France, in 1985-1986. A significant reduction in birth weight was found to be associated with an average daily alcohol consumption of three drinks or more after gestational age, infant sex, maternal age, parity, weight, and height, and cigarette smoking had been controlled for. There was no interaction between smoking and alcohol consumption on birth weight, but a significant relation between alcohol consumption and birth weight was observed among nonsmokers as well as heavy smokers. The relation observed between caffeine and birth weight disappeared after adjustment for smoking. Our results indicate that alcohol reduces birth weight, but do not support the hypothesis of an interaction between smoking and alcohol consumption.