Cefaclor is a cephalosporin antibiotic whose chemical structure is similar to that of cephalexin. The substitution of a chloro group for the methyl group of cephalexin has produced a compound with markedly improved antibacterial activity, while retaining the property of gastrointestinal absorption. Cefaclor has shown good in vitro activity against Neisseria gonorrhoeae. In a controlled clinical trial, 40 men with uncomplicated gonococcal urethritis received cefaclor given as a one gram loading dose, followed by 500 mg 4 times daily for 3 days, for a total dose of 7 g. All patients were re-evaluated at 3 to 7 days following completion of therapy. Two patients did not complete the entire course of therapy and were eventually treated with another regimen. Of 38 men who took the full course of therapy, 35 were clinically and bacteriologically cured. Two men were clinically infected but had negative pretreatment cultures. Of 19 men with beta-lactamase-positive gonococcal urethritis, 18 were cured, whereas among 17 men with penicillin-sensitive strains, all were cured. There were no adverse reactions to the drug, and all patients expressed a preference for the oral regimen. The success of cefaclor in this pilot study suggests that additional clinical trials should be performed.