This study was designed to test the hypothesis that chlorthalidone, a diuretic used to treat hypertension, depletes body zinc enough to interfere with testosterone production and thereby cause sexual dysfunction. Serum and hair zinc, serum testosterone, and sexual function were measured in 19 middle-aged hypertensive men who had been taking chlorthalidone in average daily doses of between 11 and 50 mg per day for at least 6 months, and a control group of 31 unmedicated middle-aged normotensive men. Although the medicated group had a higher incidence of sexual dysfunction (42% as compared to 16% in the control group), use of chlorthalidone did not significantly affect serum testosterone levels. When other variables were controlled for, medicated men had significantly higher serum zinc levels, and men on the highest dosage of chlorthalidone had higher hair zinc levels than all other men. There was no significant correlation between hair zinc, serum zinc, testosterone, and sexual function. Serum zinc decreased significantly with age (p = 0.043) and with ethanol intake after controlling for age (p = 0.032). Sexual dysfunction occurred more often in older men (p = 0.002). In the unmedicated controls, prevalence of dysfunction increased slightly with increasing serum potassium levels.