Illicit drug abuse, as well as smoking, are known risk factors in the development of intrauterine growth retardation. In an attempt to clarify the influence of these two aetiological factors, a retrospective analysis was carried out. 35 drug- and nicotine-addicted pregnant women were compared to 104 smokers and 101 non-smoking controls with regard to foetal outcome. Relative risk estimates for intrauterine growth retardation of the drug and nicotine group were 3.14 (95% confidence limits 1.88-5.25; p = 0.0001) whereas the smoking group had 1.39 (95% confidence limits 0.96-2.02; p = 0.0482) compared to non-smoking controls. The foetal outcome was best in the latter group: children of drug-addicted mothers had a considerably higher morbidity and mortality rate. This study shows an almost threefold risk for intrauterine growth retardation in drug-addicts who smoke, compared to nicotine abuse alone.