The investigators have preliminary data suggesting that obese patients with hypogonadotropic hypogonadism (HHG) have minimal benefit from testosterone therapy likely because of its conversion to estradiol by the abundant aromatase enzyme in the adipocytes. The increased conversion of androgens into estrogens in obese men results in a negative feedback of high estradiol levels on hypothalamus and pituitary, inhibiting the production of gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH) and follicle stimulating hormone (FSH) and, as a consequence, of testosterone by the testis. Testosterone administration could increase estradiol production, further promoting the inhibitory feedback to the hypothalamic-pituitary-gonadal axis. Although weight loss from lifestyle modification has been shown to reduce estradiol and increase testosterone levels, the effect is at best modest and weight regain results in recurrence of hypogonadism. The use of aromatase inhibitors, in combination with weight loss, could be an effective alternative strategy due to its action at the pathophysiology of the disease. Intervention Subjects (body mass index of ≥35, testosterone \<300 ng/dl) will be randomized to the active (anastrozole) or control (placebo) group. Anastrozole 1 mg tablet / day will be self-administered with or without food, at around the same time every day (active group); placebo 1 tablet/day with or without food to take at around the same time every day (control group). The study duration will be 12 months. Both groups will undergo lifestyle intervention consisting of diet and supervised exercise program. Target weight loss will be at least 10% of baseline body weight during the intervention. Subjects will attend weekly group behavior modification sessions which will last \~75-90 min for the first 3 months and decreased to every two weeks from 3 to 12 months. Subjects will attend supervised research center-based exercise sessions during the first 6 months followed by community fitness center-based sessions during the next 6 months for at least 2 d/wk, with recording of home-based exercises for the other 2-4 days/week.