A total of 157 women attending departments of genitourinary medicine were treated for chlamydial infection of the cervix with erythromycin stearate 500 mg twice a day. Chlamydiae were eradicated from the cervix in 64/80 women treated for seven days and in 51/77 women treated for 14 days. In 12 of those treated for seven days and 15 of those treated for 14 days, reinfection was the probable cause of reisolation after treatment. The possibility of latent infection with Chlamydia trachomatis could not be excluded in five women, but was not more likely to occur with the shorter treatment course. Erythromycin stearate 500 mg twice daily for seven days appears to be an effective regimen for the treatment of uncomplicated chlamydial infection of the cervix.