It has been shown in previous study that progressive glycemic deterioration was associated with progressive loss of b-cell function, measured by the decrease in plasma insulin levels, irrespective of the therapy used (diet, sulfonylureas or metformin).There is growing evidence that thiazolidinediones could have a positive action on the b-cell function. But it has not yet been demonstrated that they could protect from a deterioration in insulin secretion in the long term. So, it appears interesting to study the long term evolution of the b-cell function and the possible protection with rosiglitazone in patients with type 2 diabetes showing evidence of loss of b-cell function with metformin alone.