The aim of this study was to examine the effect of caffeine consumption during pregnancy on birth weight and its possible interaction with smoking. The sample included 1,011 women who were interviewed during their first 3 days after delivery in one of the hospitals of Belgrade, Yugoslavia. A significant reduction in birth weight was found to be associated with an average caffeine intake of > or = 71 mg per day, after adjustment for gestational age, infant sex, parity, and maternal height and weight, but only in infants born to nonsmoking mothers.