The aim of this prospective study was to evaluate the evolution of glucose tolerance (GT), insulin secretion and peripheral insulin sensitivity during a 3-yr follow-up in a homogenous population consisting of fourteen non-diabetic adults with thalassemia major (TM). All the patients underwent 2 OGTTs with a 3-yr interval and random measurements of fasting glycemia during the entire follow-up. At the time of both OGTTs, peripheral insulin sensitivity was assessed by both homeostatic model assessment (HOMA) index and a novel index derived from the OGTT. At the second OGTT patients exhibited both significantly higher fasting glucose concentrations and enhanced glycemic responses, with greater average glucose areas. GT deterioration over time was accompanied by a reduction of insulin sensitivity, with no concomitant change of insulin secretion. No patient developed diabetes mellitus (DM) during follow-up. To conclude, the natural history of glycometabolic status in TM adults seems to be characterized by a GT deterioration over time, which may probably reflect an increase of insulin resistance. GT deterioration is more evident in patients with the highest responses to the 1st OGTT and particularly in those with pre-existing impaired GT.