To evaluate if at diagnosis some factors are predictive of the insulin treatment, 75 pregnant women with gestational diabetes mellitus (GDM) and 75 with normal glucose tolerance were examined. The GDM group underwent a diet containing 25 kcal/kg/24 h and insulin was started when fasting and 2 h after meals glucose levels were respectively > 95 and > 125 mg/dl. 52 patients were treated with diet (GDd) and 23 with diet and insulin (GDi). Diagnosis week, prepregnancy body mass index (BMI), weight gain, glucose levels during oral glucose tolerance test (OGTT) (0', 60', 120' and 180'), area under curve (AUC), triglyceride levels and infant weight at delivery were evaluated. Triglyceride levels were higher in GDi respect to NGT group (232.2 +/- 110.7 mg/dl vs 147.4 +/- 70 mg/dl; p < 0.001). Diagnosis was earlier and 180' glucose levels, AUC and BMI were higher in GDi than GDd group. In conclusion, triglyceride and 180' glucose levels, AUC, pregestational BMI and diagnosis week are predictive factors for insulin treatment in gestational diabetes.