Objective: To compare the rate of adverse perinatal outcomes among women with gestational diabetes mellitus (GDM), monitored by 1 versus 2 hour-postprandial glucose (PPG) measurements. Methods: A total of 112 women diagnosed with GDM, by the criteria of Carpenter-Coustan, were included in the study population. Women were recruited from two different treatment settings, but were managed by the same team of health-care professionals using a standardized protocol. Allocation to treatment group was based on treatment setting. Glucose levels were measured fasting, and either 1 hour (1-hour monitoring group-target values <140 mg/dl) or 2 hours (2-hour monitoring group-target values <120 mg/dl) postprandially. Demographic data and perinatat outcomes were collected from their medical records. Results: In all, 66 women were assigned to 1-hour monitoring group (1 h-PPG) and 46 women to 2-hour monitoring group (2 h-PPG). There were no differences in parity, family history of diabetes, rate of GDM in previous pregnancies, weight gain, pregestational BMI and 50-g-glucose challenge test (GCT) and 100-g oral glucose challenge test (OGTT) results. As expected, there was a significant difference in mean blood glucose levels between the two groups (108.1±19.2 and 94.9±21.2 mg/dl, 1- and 2 hours, respectively, p<0.0001); however, HbA1C levels were similar in the two groups. Perinatal outcomes were defined as gestational week at delivery; fetal weight (3325±471 vs 3309±608 g, respectively) and percentile (47.2±27 vs 49.6±30, respectively), and were similar for both groups. Insulin therapy was initiated more frequently in 2-hour monitoring group (28 and 40% of women in groups 1 and 2, respectively; p<0.05). Rates of macrosomia (7.5 versus 10.6%), large for gestational age (7.4 versus 15.2%), and delivery by cesarean section (24 versus 30%) were increased in group 2 (2 h-PPG) but these differences did not reach statistical significance. Conclusion: These data suggest that diet control in women with GDM managed by 1-hour PPG measurements is associated with a decreased rate of insulin therapy. However, neonatal and obstetrical outcomes are not determined by the timing of their glucose determinations.