Cefaclor, a new orally administered cephalosporin, was evaluated by a randomized trial for effectiveness in the treatment of uncomplicated urethritis due to Neisseria gonorrhoeae in men. Regimens included 2,3, and 4 g of cefaclor, with or without 1 g of orally administered probenecid, as single daily doses for three days. The diagnoses were confirmed by isolation of N. gonorrhoeae; cures or therapeutic failures were determined by follow-up cultures on day 7 after completion of therapy. Sixty-six (73%) of 90 treated patients were evaluable for efficacy. The bacteriologic cure rate was 98% (65/66); one patient treated with 2 g of cefaclor plus probenecid had a positive culture for N. gonorrhoeae on follow-up examination. Adverse reactions consisted of mild nausea in five patients (7%) and vomiting in one patient (1%) who received 3- or 4-g doses. No treatment was discontinued, and no abnormality of screening hematologic tests or enzymes was observed. Thus, cefaclor, given in multiple doses, was highly efficacious for treatment of uncomplicated gonococcal urethritis in men.