Objectives: Erectile dysfunction (ED) and androgen deficiency in aging men are two separate clinical entities that often overlap. Controversy exists regarding the most appropriate total testosterone level that defines androgen deficiency in aging men, and its prevalence in men with ED is still uncertain. We evaluated the prevalence and risk factors of low and low-normal testosterone levels in men presenting for an initial ED evaluation. Methods: The computerized charts from 1987 to 2002 of 2794 men aged 25 to 80 years and presenting with a primary complaint of ED who also had serum total testosterone levels measured were retrospectively reviewed. Multiple testosterone level cutpoints and a linear regression model (including age, diabetes, cholesterol, anemia, creatinine, and prostate-specific antigen) were used to analyze the factors that correlated with hypogonadism. Results: The prevalence of androgen deficiency was 7%, 23%, 33%, and 47% for testosterone levels of less than 200, less than 300, less than 346, and less than 400 ng/dL, respectively. An abrupt increase in hypogonadism prevalence occurred in men aged 45 to 50, beyond which a plateau of prevalence was maintained until older than 80 years of age. Age, the presence of uncontrolled diabetes, high total cholesterol, and anemia all correlated with significantly decreased testosterone levels in men with ED. The prostate-specific antigen level and creatinine did not affect the testosterone levels. Conclusions: Androgen deficiency was quite common in men presenting with ED and correlated significantly with age, uncontrolled diabetes, hypercholesteremia, and anemia. Although additional prospective studies evaluating the effect of testosterone supplementation in this population are needed, clinicians, including urologists, should be keenly aware of the large overlap of patients with ED who might also have the entity, androgen deficiency in the aging male.