OBJECTIVES: Metformin is recognized as the treatment of chronic obese, insulin-resistant type 2 diabetic patients. Whether it improves insulin sensitivity in obese patients with normal glucose tolerance remains unknown. METHODS: Eight obese female patients with normal glucose tolerance were studied during a double blinded, randomized cross-over study including a 2-week administration of metformin and a 2-week administration of placebo. Insulin secretion and insulin sensitivity were assessed after metformin and placebo by means of a 3-hour hyperglycemic clamp. RESULTS: The plasma insulin and C-peptide concentrations during the hyperglycemic clamp were identical after placebo or metformin (both first and second phases). Insulin-mediated glucose disposal, stimulation of glucose oxidation and suppression of endogenous glucose production were identical after metformin and placebo. CONCLUSIONS: Metformin does not improve insulin sensitivity nor insulin secretion in obese female patients with normal glucose tolerance.