OBJECTIVE To study the dose response to metformin in type II diabetic patients. RESEARCH DESIGN AND METHODS Type II diabetic patients with a BMI > 25 were treated with 3,000 mg/day (n = 27), 1,500 mg/day (n = 25), or placebo (n = 23) for 6 months. Venous blood samples were taken at each visit for plasma glucose and insulin, HbA1c, triglyceride and cholesterol, plasminogen activator inhibitor-1 (PAI-1) antigen and activity, tissue plasminogen activator (TPA), and euglobulin clot lysis time (ECLT). Blood pressure was recorded at each visit. RESULTS There were no changes in BMI or blood pressure. Blood glucose fell (mean) by 3.6 mmol/l in the high-dose and 0.5 mmol/l in the low-dose group over the 6-month study (P < 0.001 and NS compared with placebo). HbA1c and plasma insulin fell in both treatment groups (HbA1c, P < 0.001; insulin, P < 0.003 and 0.03). There was a fall in triglyceride (P < 0.05) and cholesterol (P < 0.008) with high-dose metformin. PAI-1 antigen and activity fell by ∼ 20% of baseline in both treatment groups (PAI-1 antigen high dose, P < 0.01; PAI-1 antigen low dose, P < 0.002: PAI-1 activity high and low dose, P < 0.003). There were significant falls in total tPA in both groups (P < 0.004), but the overall effect was a fall in ECLT (P < 0.03). CONCLUSIONS The results indicate that metformin has favorable effects on cardiovascular risk factors associated with type II diabetes. The effects on glycemic control and lipids are dose-dependent, while the enhanced fibrinolytic response is independent of the doses used.