Introduction. Determinants of erectile dysfunction in diabetic men have not been adequately investigated as potential mediators of change. Aim. To determine the prevalence and correlates of erectile dysfunction (ED) in overweight men with type 2 diabetes in the multicenter, Look AHEAD trial (Action for Health in Diabetes). Main Outcome Measures. International Index of Erectile Function (IIEF), self-reported use of phosphodiesterase type 5 inhibitors, laboratory measures of adiposity, cardiometabolic parameters, and exercise fitness. Methods. Male participants aged 45-75 in the Look AHEAD trial in a committed relationship were recruited for an ongoing study of sexual function and diabetes. Eligible participants completed the IIEF questionnaire and provided updated information on use of medical treatments for sexual dysfunction. Baseline sexual function results for participants in the male ancillary study are reported here; intervention data and results for female participants are presented elsewhere. Results. A total of 373 eligible male participants completed all sexual function questionnaires, of whom 263 (68.7%) were sexually active at the time of the study. Almost half (49.8%) of the men reported mild or moderate degrees of ED, and 24.8% had complete ED. Among sexually active participants, 42.6% had sought medical help for their problem, and 39.7% reported use of ED medications. ED was significantly associated with age (odds ratio [OR] = 1.05; confidence interval [CI]: 1.01-1.10) baseline HbA 1c (OR = 1.31; CI: 1.05-1.63), hypertension history (OR=2.41; CI: 1.34-4.36), and metabolic syndrome (OR = 3.05, CI: 1.31-7.11). Of note, cardiorespiratory fitness was found to be protective of ED in a multivariable analysis (OR=0.61; P < 0.001). Conclusions. ED is prevalent in this sample of obese, type 2 diabetic men in the Look AHEAD study. Cardiovascular risk factors were highly associated with ED in this population, and cardiorespiratory fitness was protective in this analysis. © 2009 International Society for Sexual Medicine.