In a multicentre trial in general practice, a total of 1823 type 2 diabetics, not adequately controlled by diet and maximal sulphonylurea therapy, were treated with additional metformin 850--2 550 mg/d for 12 weeks. The average postprandial blood sugar decreased from 15.48 mmol/l to 10.43 mmol/l, HbA1 fell from 11.0% to 9.1%. Serum triglycerides decreased from an initial level of 2.87 mmol/l to 2.41 mmol/l an mean total cholesterol from 6.76 mmol/l to 6.16 mmol/l. In addition, body weight and blood pressure declined steadily. All the described changes were statistically significant. The metformin-sulphonylurea combination therapy was generally well tolerated. Gastrointestinal side effects occurred in about 7% of patients, but mainly in the first week of treatment, generally disappearing spontaneous as therapy was continued. Side effects led to a discontinuation of the treatment in 4.2% of patients.