OBJECTIVE: To evaluate the safety and efficacy of closed-loop insulin delivery in well-controlled pregnant women with type 1 diabetes treated with continuous subcutaneous insulin infusion (CSII). RESEARCH DESIGN AND METHODS: A total of 12 women with type 1 diabetes (aged 32.9 years, diabetes duration 17.6 years, BMI 27.1 kg/m(2), and HbA(1c) 6.4%) were randomly allocated to closed-loop or conventional CSII. They performed normal daily activities (standardized meals, snacks, and exercise) for 24 h on two occasions at 19 and 23 weeks' gestation. Plasma glucose time in target (63-140 mg/dL) and time spent hypoglycemic were calculated. RESULTS: Plasma glucose time in target was comparable for closed-loop and conventional CSII (median [interquartile range]: 81 [59-87] vs. 81% [54-90]; P = 0.75). Less time was spent hypoglycemic (<45 mg/dL [0.0 vs. 0.3%]; P = 0.04), with a lower low blood glucose index (2.4 [0.9-3.5] vs. 3.3 [1.9-5.1]; P = 0.03), during closed-loop insulin delivery. CONCLUSIONS: Closed-loop insulin delivery was as effective as conventional CSII, with less time spent in extreme hypoglycemia.