OBJECTIVE - Strategies for preventing hypoglycemia during exercise in children with type 1 diabetes have not been well studied. The Diabetes Research in Children Network (DirecNet) Study Group conducted a study to determine whether stopping basal insulin could reduce the frequency of hypoglycemia occurring during exercise. RESEARCH DESIGN AND METHODS - Using a randomized crossover design, 49 children 8-17 years of age with type 1 diabetes on insulin pump therapy were studied during structured exercise sessions on 2 days. On day 1, basal insulin was stopped during exercise, and on day 2 it was continued. Each exercise session, performed from ∼4:00-5:00 P.M., consisted of four 15-min treadmill cycles at a target heart rate of 140 bpm (interspersed with three 5-min rest breaks over 75 min), followed by a 45-min observation period. Frequently sampled glucose concentrations (measured in the DirecNet Central Laboratory) were measured before, during, and after the exercise. RESULTS - Hypoglycemia (≤70 mg/dl) during exercise occurred less frequently when the basal insulin was discontinued than when it was continued (16 vs. 43%; P = 0.003). Hyperglycemia (increase from baseline of ≥20% to ≥200 mg/dl) 45 min after the completion of exercise was more frequent without basal insulin (27 vs. 4%; P = 0.002). There were no cases of abnormal blood ketone levels. CONCLUSIONS - Discontinuing basal insulin during exercise is an effective strategy for reducing hypoglycemia in children with type 1 diabetes, but the risk of hyperglycemia is increased.