In a double-blind comparative trial, 100 men with uncomplicated gonorrhea caused by β-lactamase-negative Neisseria gonorrhoeae were treated with a single 0.25-g dose of ciprofloxacin administered orally or with 3.5 g of ampicillin plus 1.0 g of probenecid administered orally. Urethral infection was eradicated in all 49 men treated with ciprofloxacin and in 47 (92%) of 51 men treated with ampicillin-probenecid (P = 0.12). The geometric mean MICs for pretreatment isolates were 0.008 μg of ciprofloxacin per ml, 0.09 μg of penicillin G per ml, 0.52 μg of tetracycline per ml and 23.5 μg of spectinomycin per ml. Chlamydia trachomatis infection persisted in 10 of 11 men treated with ciprofloxacin and in 11 of 14 men treated with ampicillin-probenecid. A single 0.25-g dose of ciprofloxacin was effective for treatment of uncomplicated urethral gonorrhea in men, but it did not eradicate coinfection with C. trachomatis.