BACKGROUND Healthy diet, physical activity and modest weight gain during pregnancy may prevent developing gestational diabetes mellitus (GDM). We examined whether a lifestyle intervention designed to prevent GDM was effective in reducing excessive gestational weight gain (GWG). METHODS A cluster-randomised controlled trial (n=399) was conducted in maternity clinics in 14 municipalities in Southern Finland. Pregnant women with at least one risk factor for GDM (for example, overweight) but no pre-existing diabetes were recruited at 8-12 weeks' gestation. The intervention included counselling on GWG, physical activity and healthy eating at five routine visits. Usual counselling practices were continued in the usual care municipalities. Statistical analyses were performed using multilevel linear and logistic regression models adjusted for weeks' gestation at last weight measurement, pre-pregnancy body mass index and smoking status. RESULTS The intervention group had a lower mean GWG by weeks' gestation than the usual care group (adjusted coefficient for the between-group difference -0.016 kg per day, P=0.041). There was no difference in mean (± s.d.) GWG between the intervention and the usual care groups (13.7 ± 5.8 vs 14.3 ± 5.0 kg, P=0.64). In total, 46.8% of the intervention group and 54.4% of the usual care group exceeded the GWG recommendations. The adjusted odds ratio for excessive GWG was 0.82 (95% CI 0.53-1.26, P=0.36) in the intervention group as compared with the usual care group. CONCLUSIONS The intervention had minor effects on GWG among women who were at increased risk for GDM. In order to prevent excessive GWG, additional focus on restriction of energy intake may be needed.