Aim: To evaluate the treatment effect in women with gestational diabetes according to the IADPSG criteria. Methods: We prospectively studied a cohort of 1050 consecutive pregnant women undergoing an universal screening for GDM by applying the IADPSG criteria (now accepted and recommended by the WHO and FIGO). Maternal and neonatal outcomes between the treated and untreated group are compared. Results: The prevalence of GDM in our cohort was 23.8% (250 women with 8 patients lost to follow up) following diagnostic criteria of HAPO 5. Of these 182 were treated by the endocrinological department. Treatment consisted of a diet of 1800 kcal/day in combination with physical exercise. Insulin therapy was initiated in 6 women. Due to various reasons, 60 women were untreated. Maternal and neonatal outcomes between the two groups were compared. The outcomes consisted of maternal (weight gain, spontaneous delivery, induction of labor, etc.) and neonatal (birth weight, apgar scores, etc.) outcomes and there were no statistically nor clinically significant differences between the groups. A subanalysis of pregnant women with a BMI >30 provided insufficient data to make any conclusions (48 patients and only 8 of them did not receive therapy). By applying the HAPO 4 criteria, the prevalence of GDM was 16.9% (177 women with 8 patients lost to follow up), and, although not reaching statistical significance, there is a clear trend of therapy efficiency in this patient group. Conclusions: Screening for and treatment of GDM remains controversial. Our ongoing study shows that by an universal application of the IADPSG criteria the prevalence of GDM doubles or even triples in our population. Because our number of events are low we cannot yet make firm conclusions on the effect of treatment on important clinical outcomes and more data are needed. Moreover, looking further to the optimal cut off values of diagnostic testing remains of utmost importance in further research.