Objective: Metabolic control often deteriorates during puberty in children with type 1 diabetes. The aim of the present study was to investigate whether addition of metformin for 3 months improves metabolic control and insulin sensitivity. Design: Twenty-six of 30 randomised adolescents with type 1 diabetes (18 females, eight males) completed a double-blind placebo-controlled trial. Their mean age was 16.9^1.6 (S.D.) years, mean glycated haemoglobin (HbA 1c ) 9.5^1.1% and daily insulin dosage 1.2^0.3 U/kg. The participants were randomised to receive oral metformin or placebo for 3 months. HbA 1c was measured monthly, and peripheral insulin sensitivity was assessed by a euglycaemic hyperinsulinaemic clamp at baseline and at the end of the study. Results: HbA 1c decreased significantly in the group treated with metformin, from 9.6 to 8.7% (P , 0.05), but was unchanged in the placebo group (9.5 vs 9.2%). Peripheral glucose uptake divided by mean plasma insulin concentration was increased in the metformin group (P , 0.05) but not in the placebo group. Initial insulin sensitivity was inversely correlated to changes in HbA 1c (r ¼ 20.62; P , 0.05) and positively correlated to changes in insulin sensitivity (r ¼ 0.77; P , 0.01). Conclusions: In this double-blind placebo-controlled study we found that metformin improves metabolic control in adolescents with type 1 diabetes. The effect seems to be associated with an increased insulin-induced glucose uptake.