In a population based cohort study information on the consumption of alcohol was obtained from 95% of the 952 consecutive primigravidas who lived in the Dundee district and attended for antenatal care between May 1985 and April 1986. Before realising that they were pregnant more than 90% drank alcohol and 53% were cigarette smokers. During the first four months of pregnancy, however, the proportion of women drinking and smoking fell to 56% and 44%, respectively. Alcohol consumption of more than 120 g absolute alcohol/week (12 or more standard drinks) during pregnancy was related to shorter gestational age (-2.6 weeks), smaller head circumference (-18 mm), shorter (-21 mm) and lighter (-499 g) babies, and lower Apgar scores at five minutes (-0.4, all p less than 0.01). After adjustment for the effect of smoking, social class, mother's size, and other confounding factors, however, an alcohol intake of more than 120 g/week was significantly related only to shorter gestational age (-2.0 weeks, p less than 0.001) and lower Apgar score at five minutes (-0.2, p less than 0.05). Alcohol intake in the region of 100-119 g/week was significantly related to smaller head circumference (-12 mm, p less than 0.05). Analysis by type of beverage consumed suggested that beer rather than wine or spirits was associated with a poorer outcome. As there was no detectable effect on pregnancy of alcohol consumption below 100 g/week, it is suggested that health education should be directed towards mothers who drink more than this amount.