BACKGROUND: In addition to its glucose-lowering effect, metformin treatment has been suggested to improve lipidaemia in patients with type 2 diabetes. In contrast, in patients with type 1 diabetes (T1DM), information about the effect of metformin treatment on lipidaemia is limited. In this study, we report the effect of a 1-year treatment with metformin vs. placebo on plasma lipids in T1DM patients and persistent poor glycaemic control. METHODS: One hundred T1DM patients with haemoglobinA(1c) (HbA(1c)) > or =8.5% during the year before enrolment entered a 1-month run-in period on placebo treatment. Thereafter, patients were randomized (baseline) to treatment with either metformin (1000 mg twice daily) or placebo for 12 months (double masked). Patients continued ongoing insulin therapy and their usual outpatient clinical care. Outcomes were assessed at baseline and after 1 year. RESULTS: After 1 year, in those patients who did not start or stop statin therapy during the trial, metformin treatment significantly reduced total and LDL cholesterol by approximately 0.3 mmol/l compared with placebo (p = 0.021 and p = 0.018 respectively). Adjustment for statin use or known cardiovascular disease did not change conclusions. In statin users (metformin: n = 22, placebo: n = 13), metformin significantly lowered levels of LDL and non-HDL cholesterol by approximately 0.5 mmol/l compared with placebo (adjusted for changes in statin dose or agent: p = 0.048 and p = 0.033 respectively). HbA(1c) (previously reported) was not significant different between treatments. CONCLUSION: In patients with poorly controlled T1DM, at similar glycaemic levels, adjunct metformin therapy during 1 year significantly lowered levels of proatherogenic cholesterolaemia independent of statin therapy.