A multi-centre, non-randomized clinical study of 12-months' duration was performed in 112 patients with hyperlipidaemia associated with non-insulin-dependent diabetes mellitus to evaluate the clinical efficacy and tolerability of pravastatin and to assess its effect on glycaemic control. Patients were eligible for this trial if they fulfilled the following criteria; a high plasma total cholesterol level greater than 220 mg/dl associated with stable glycaemic control for at least 3-months' observation period. On entry, patients received 10 mg pravastatin per day (5 mg twice daily) for 12 months. Clinical efficacy was evaluated in 108 patients. The results showed that pravastatin induced a significant decrease in serum cholesterol level mainly with LDL-cholesterol. Total cholesterol levels were decreased significantly from 275 +/- 3 mg/dl to 222 +/- 4 mg/dl within 3 months of the start of treatment, and LDL-cholesterol decreased from 192 +/- 4 mg/dl to 137 +/- 4 mg/dl. After 12 months' treatment, total cholesterol and LDL-cholesterol levels were 216 +/- 4 mg/dl (p < 0.001) and 137 +/- 5 mg/dl (p < 0.001), respectively. HDL-cholesterol levels were increased from 51 +/- 2 mg/dl to 56 +/- 2 mg/dl at 3 months (p < 0.001) and 55 +/- 2 mg/dl at 12 months (p < 0.01). Triglyceride concentrations were also decreased from 173 +/- 11 mg/dl to 156 +/- 13 mg/dl at 3 months and 137 +/- 10 mg/dl at 12 months (p < 0.01). Fasting plasma glucose and glycated haemoglobin levels were not affected by pravastatin. Adverse events observed in 5 cases were always mild and reversible. These results indicate a clinical usefulness of pravastatin with high compliance in patients with hyperlipidaemia associated with non-insulin-dependent diabetes mellitus.