Background: Despite intensive insulin treatment, many patients with type-1 diabetes (T1DM) have longstanding inadequate glycaemic control. Metformin is an oral hypoglycemic agent that improves insulin action in patients with type-2 diabetes. We investigated the effect of a one-year treatment with metformin versus placebo in patients with T1DM and persistent poor glycaemic control. Methodology/Principal Findings: One hundred patients with T1DM, preserved hypoglycaemic awareness and HaemoglobinA1c (HbA1c) ≥ 8.5% during the year before enrolment entered a one-month run-in on placebo treatment Thereafter, patients were randomized (baseline) to treatment with either metformin (1 g twice daily) or placebo for 12 months (double-masked). Patients continued ongoing insulin therapy and their usual outpatient clinical care, The primary outcome measure was change in HbA1c after one year of treatment. At enrolment, mean (standard deviation) HbA1c was 9.48% (0.99) for the metformin group (n=49) and 9.60% (0.86) for the placebo group (n=51). Mean (95% confidence interval) baseline-adjusted differences after 12 months with metformin (n=48) versus placebo (n=50) were: HbA1c, 0.13% (-0.19; 0.44), p=0.422; Total daily insulin dose, -5.7 U/day (-8.6; -2.9), p<0.001; body weight, -1.74 kg (-3.32; -0.17), p=0.030. Minor and overall major hypoglycemia was not significantly different between treatments. Treatments were well tolerated. Conclusions/Significance: In patients with poorly controlled T1DM, adjunct metformin therapy did not provide any improvement of glycaemic control after one year. Nevertheless, adjunct metformin treatment was associated with sustained reductions of insulin dose and body weight. Further investigations into the potential cardiovascular-protective effects of metformin therapy in patients with T1DM are warranted.