As part of a feasibility study to prevent gestational diabetes mellitus (GDM), we evaluated the effect of an intensive dietary therapy on quality of diet, weight gain and birth weight in women at high risk of GDM. Women with risk factors for GDM (n 54) were randomly assigned from April 2005 to May 2006 to a lifestyle intervention group (n 27) including dietary advice six times during pregnancy or to a close follow-up group (n 27) in a community-based setting in Finland. Dietary intake was recorded three times during pregnancy using 4 d food records. The main outcome was the incidence of GDM. The secondary outcomes were the changes in nutrient intake, weight gain and birth weight. Overall, seventeen (65 %) women in the intervention group and eighteen (69 %) women in the close follow-up group returned all three food records. PUFA intake increased (P = 0·008) during pregnancy in the intervention as compared to the close follow-up group. There were no clear differences in the changes of saturated fat or fibre intake between the groups. Intensive dietary education resulted in a somewhat lower weight gain during pregnancy (P = 0·062) and higher birth weights of the infants (P = 0·047) without an effect on macrosomia as compared to the close follow-up group. Individualised counselling by a clinical nutritionist as part of a lifestyle intervention improved the quality of dietary fat intake in pregnant women at high risk of GDM.