BACKGROUND Combination oral therapy is often used to control the hyperglycemia of patients with type 2 diabetes. We compared the effectiveness of metformin and troglitazone when added to sulfonylurea therapy for patients with type 2 diabetes who had suboptimal blood glucose control. METHODS We used a randomized 2-group design to compare the efficacy, safety, and tolerability of troglitazone and metformin for patients with type 2 diabetes mellitus that was inadequately controlled with diet and oral sulfonylureas. Thirty-two subjects were randomized to receive either troglitazone or metformin for 14 weeks, including a 2-week drug-titration period. The primary outcome variable was mean change in the level of glycosylated hemoglobin (Hb A1c) from baseline. Secondary outcomes included mean changes from baseline in fasting plasma glucose and C-peptide levels, renal or metabolic side effects, and symptomatic tolerability. RESULTS The addition of either troglitazone or metformin to oral sulfonylurea therapy significantly decreased Hb A1c levels. Both treatment regimens also significantly reduced fasting plasma glucose and C-peptide levels. We found no significant differences between the treatment arms in efficacy, metabolic side effects, or tolerability. CONCLUSIONS Our results demonstrate that troglitazone and metformin each significantly improved Hb A1c, fasting plasma glucose, and C-peptide levels when added to oral sulfonylurea therapy for patients with type 2 diabetes who had inadequate glucose control.