The thiazolidinedione rosiglitazone and the biguanide metformin are effective antihyperglycaemic agents with different modes of action; rosiglitazone primarily increases insulin sensitivity, whereas metformin primarily reduces hepatic glucose output. Antihyperglycaemic combination therapy is often required to achieve effective glycaemic control. A fixed-dose formulation of rosiglitazone/metformin was recently approved in the EU and the US for the treatment of type 2 diabetes mellitus in patients inadequately controlled on metformin monotherapy. Bioequivalence between the fixed-dose combination tablet and coadministration of rosiglitazone with metformin at the same dosage has been established in a pharmacokinetic study. Fixed-dose rosiglitazone/metformin 8 mg/2g per day reduced glycosylated haemoglobin (HbA1c) and fasting plasma glucose (FPG) levels to a significantly greater extent than metformin 3 g/day in patients with type 2 diabetes in a 24-week, randomised, double-blind study. Rosiglitazone plus metformin was significantly more effective than metformin alone at reducing HbA1c and FPG levels in patients with type 2 diabetes in three 26-week, randomised, double-blind, placebo-controlled studies. Rosiglitazone plus metformin was generally well tolerated in all studies and had a tolerability profile similar to that of metformin monotherapy. Mild or moderate symptomatic hypoglycaemia was reported in <or=4.4% of rosiglitazone plus metformin recipients.