In this multicentre, randomized, double-blind, controlled, parallel-group trial, 103 patients with type 2 diabetes mellitus and metabolic syndrome were randomized to receive one of two thiazolidinediones--pioglitazone or rosiglitazone--in combination with 1500 mg/day of metformin, increasing up to 3000 mg/day, for 12 months. Anthropometric, metabolic, coagulation and fibrinolysis parameters were assessed at baseline and after 3, 6, 9 and 12 months. Significant decreases in glycosylated haemoglobin, fasting plasma glucose and post-prandial plasma glucose levels were seen after 9 and 12 months in both groups, and significant decreases in fasting plasma insulin and post-prandial plasma insulin levels were seen after 12 months in both groups. In both groups, improvement in the homeostasis model assessment index compared with baseline was obtained only after 12 months. Plasminogen activator inhibitor-1 levels were significantly lower in both groups after 12 months compared with baseline values. In patients with type 2 diabetes mellitus and metabolic syndrome, the combination of metformin plus thiazolidinediones improved glycaemic control and produced a slight but significant reduction in plasminogen activator inhibitor-1 levels.