Summary Background Closed-loop systems link continuous glucose measurements to insulin delivery. We aimed to establish whether closed-loop insulin delivery could control overnight blood glucose in young people. Methods We undertook three randomised crossover studies in 19 patients aged 5–18 years with type 1 diabetes of duration 6·4 years (SD 4·0). We compared standard continuous subcutaneous insulin infusion and closed-loop delivery (n=13; APCam01); closed-loop delivery after rapidly and slowly absorbed meals (n=7; APCam02); and closed-loop delivery and standard treatment after exercise (n=10; APCam03). Allocation was by computer-generated random code. Participants were masked to plasma and sensor glucose. In APCam01, investigators were masked to plasma glucose. During closed-loop nights, glucose measurements were fed every 15 min into a control algorithm calculating rate of insulin infusion, and a nurse adjusted the insulin pump. During control nights, patients' standard pump settings were applied. Primary outcomes were time for which plasma glucose concentration was 3·91–8·00 mmol/L or 3·90 mmol/L or lower. Analysis was per protocol. This trial is registered, number ISRCTN18155883. Findings 17 patients were studied for 33 closed-loop and 21 continuous infusion nights. Primary outcomes did not differ significantly between treatment groups in APCam01 (12 analysed; target range, median 52% IQR 43–83 closed loop vs 39% 15–51 standard treatment, p=0·06; ≤3·90 mmol/L, 1% 0–7 vs 2% 0–41, p=0·13), APCam02 (six analysed; target range, rapidly 53% 48–57 vs slowly absorbed meal 55% 37–64, p=0·97; ≤3·90 mmol/L, 0% 0–4 vs 0% 0–0, p=0·16), and APCam03 (nine analysed; target range 78% 60–92 closed loop vs 43% 25–65 control, p=0·0245, not significant at corrected level; ≤3·90 mmol/L, 10% 2–15 vs 6% 0–44, p=0·27). A secondary analysis of pooled data documented increased time in the target range (60% 51–88 vs 40% 18–61; p=0·0022) and reduced time for which glucose concentrations were 3·90 mmol/L or lower (2·1% (0·0–10·0) vs 4·1% (0·0–42·0); p=0·0304). No events with plasma glucose concentration lower than 3·0 mmol/L were recorded during closed-loop delivery, compared with nine events during standard treatment. Interpretation Closed-loop systems could reduce risk of nocturnal hypoglycaemia in children and adolescents with type 1 diabetes. Funding Juvenile Diabetes Research Foundation; European Foundation for Study of Diabetes; Medical Research Council Centre for Obesity and Related Metabolic Diseases; National Institute for Health Research Cambridge Biomedical Research Centre.