Tetracycline hydrochloride, 500 mg orally four times a day for seven days, remains the treatment of choice for C trachomatis infections in men and nonpregnant women. Either erythromycin, 500 mg orally four times daily for seven days, or an equivalent dosage of another erythromycin product is an alternative treatment for patients who cannot tolerate tetracycline and for pregnant women. These two treatment regimens can be generalized to include nongonococcal urethritis and mucopurulent cervicitis. However, other treatment regimens that are effective against C trachomatis may not be effective for treating nongonococcal urethritis or mucopurulent cervicitis not caused by C trachomatis. The optimal treatment for pregnant women with C trachomatis infections and women with acute PID has not been established. Additional treatment trials with both groups of patients are needed to determine the effectiveness of antimicrobial agents in addition to those currently used, to establish the appropriate dose of each antimicrobial agent, and to clarify the appropriate duration of treatment. All individuals who are sexual partners of patients with nongonococcal urethritis, mucopurulent cervicitis, and acute PID (within the 30 days prior to onset of their symptoms or time of positive clinical evaluation findings) should be examined for sexually transmitted disease and treated promptly with a regimen effective against uncomplicated gonorrhea and chlamydial infections. Prompt treatment of sexual partners reduces the rate of treatment failure due to reinfection, reduces the transmission of infection, and reduces the frequency of occurrence of adverse sequelae of infection.