A randomized double-blind trial of 152 men with gonococcal urethritis compared the therapeutic efficacy of erythromycin estolate and erythromycin base. Twenty-one of 86 (24%) men treated with the estolate and 15 of 66 (23%) treated with the base had recurrent or persistent gonococcal infection when seen after a 9-g course of erythromycin. The serum erythromycin activity among estolate-treated patients (3.57 +/- 0.84 microgram/ml) was nearly twice that for base-treated patients (1.76 +/- 0.80 microgram/ml). Our findings do not support routine use of erythromycin for treatment of pregnant, penicillin-allergic patients.