Pioglitazone may cause fluid retention, a well-known side effect of thiazolidinediones, and may exacerbate heart failure. Patients with type 2 diabetes and mild cardiac disease (New York Heart Association functional class I) received pioglitazone (n=151) or glyburide (n=149) for 1 year. The primary endpoint was change in distance covered in the 6-minute walk test. Main secondary endpoints included comparison of cardiovascular mortality and morbidity, analysis of changes from baseline in cardiac structure and function by echocardiogram, and lipid panel. There was no significant treatment difference in the mean change from baseline in the 6-minute walk test (−11.7 m [95% confidence interval, −29.79 to 6.42]). Cardiovascular mortality and morbidity were not significantly different between the treatment groups. Echocardiographic data suggested no significant deterioration in cardiac function with pioglitazone, although more heart failure (10 vs 7 patients), edema (21.2% vs 12.8%), and weight gain (2.56±4.62 kg vs 0.86±3.85 kg) were observed than with glyburide. Congest Heart Fail. 2010;16:111–117. © 2010 Wiley Periodicals, Inc.