Summary: Analysis of cigarette smoking and alcohol consumption patterns recorded in pregnancy could not detect a harmful effect of alcohol at levels below 2 glasses a day. Total abstainers fared very slightly worse than those who drank occasionally. Smoking was associated with lower birth-weight and a worse fetal outcome independent of socioeconomic status, parity, maternal age or alcohol consumption. There was evidence of an interaction between smoking and adverse social and parity factors in low birth-weight.