BACKGROUND AND OBJECTIVE Smoking and diabetes mellitus (DM) are both risk factors for complications during pregnancy and disorders in the newborn. Smoking behavior during pregnancy was investigated in German families where either the pregnant woman or her spouse had diabetes. The relationship between maternal smoking during pregnancy, her metabolic state and the child's birth weight was analysed. PATIENTS AND METHODS Data on smoking behavior during pregnancy of women and their spouses were obtained by questionnaire. 2 498 families were included: 1,439 women and 1,010 men with type 1 diabetes (T1D) and 1,059 women and 1,488 men without T1D, deliveries having taken place between 1989 and 2005 (no account was taken of the amount smoked or whether stopped during pregnancy). Also collected were data on 308 women with gestational diabetes (GDM), who delivered a baby between 1989 and 1997. RESULTS Significantly more pregnant women with T1D and GDM smoked than pregnant women without diabetes (13.2% and 15.2% vs 8.7%, p=0.001). Children of smoking pregnant women with T1D had a significantly lower birth weight than those of non-smoking pregnant women with T1D (median 54th percentile vs 71st percentile, p<0.001), although smoking pregnant women had a higher HbA (1c) than non-smoking pregnant women with T1D (median 6.1% vs 5.7%, p<0.001). The number of women and men who smoked during pregnancy declined between 1989 and 2005 (women: 17.5% vs. 8.9%, p=0.02, men: 38.0% vs 24.7%, p<0.001). CONCLUSIONS The lower birth weight of children of mothers who smoked demonstrates the detrimental consequences of smoking during pregnancy. Pregnant woman with diabetes are a high risk group that should be targeted with programmes that aim to stop smoking during pregnancy.