Introduction. Gestational diabetes mellitus (GDM) increases the risk for diabetes in the next pregnancy and later in life. Thus, estimating the risk of GDM in further pregnancies provides a time frame for possible preventive measures. We aimed to calculate the recurrence rate of GDM in primiparous women and evaluate the factors involved such as age, body mass index, weight gain, time between pregnancy and postpartum OGTT results. Material and methods. We established a prospective cohort during a 5-year period at the Department of Obstetrics at Kolding Hospital. Women with diet-treated GDM in their first pregnancy and a subsequent pregnancy constituted our study population. Multiparity and insulin-treated GDM were exclusion criteria. Results. Among 15 735 deliveries, 535 women were diagnosed with GDM (3.4%). Of these, 209 (39.1%) were nulliparous women, treated with diet only. Seventy-two of these women had a subsequent pregnancy and they all attended the recommended screening procedure, a 75-g oral glucose tolerance test at 14-20 (early) or 27-30 (late) weeks' gestation. The recurrence rate of GDM was 47.2%. The risk of recurrence was less in women who lost weight between the first and the subsequent pregnancy. Conclusions. Recurrence of diet-treated GDM was 47.2% in primiparous women with previous GDM and the recurrence was associated with weight gain between pregnancies.