Pioglitazone is an insulin-sensitizer with a thiazolidinedione structure. It is used to reduce hyperglycemia and is frequently prescribed to type 2 diabetic patients. However, it causes edema as an adverse effect in some patients. Although the mechanism of edema is unclear, it may bring an increased risk of congestive heart failure. We investigated whether pioglitazone correlates with the level of plasma human atrial natriuretic peptide (hANP), a marker for congestive heart failure. We administered 15 mg/day of pioglitazone for 3 months to 49 patients (34 men and 15 women; mean age: 64+/-12 years) with type 2 diabetes and no history of pretibial edema. Three of the patients complained of pretibial edema during the 3-month period, and their plasma hANP levels were higher than those of the other 46 before and during the treatment. We therefore suspect that pretibial edema appearing after administration of low-doses of pioglitazone coincides with the level of plasma hANP, and that the appearance of pretibial edema may reflect an increase in circulating blood volume induced by pioglitazone.