Three patients, 1 man and 2 women, aged 60, 55 and 72 years, had an insufficient glucose regulation with insulin therapy or with sulfonylurea derivatives. They started metformin therapy, after which HbA1c decreased by 0.8-1.8%. One patient had to discontinue metformin therapy due to excessive diarrhoea. Many studies have shown the beneficial effect of metformin on glucose control. The recent UK Prospective Diabetes Study has proven the effectiveness of metformin for any diabetes related endpoint. Frequent side effects of metformin are nausea, abdominal discomfort and diarrhoea. Most side effects disappear after decreasing the dosage, although in 5% of patients diarrhoea only disappears after discontinuation of metformin. Lactic acidosis is a rare, serious adverse effect of metformin, which can be prevented by carefully observing the contra-indications.