The incidence of chlamydial infections was studied in 33 male and 48 female regular sexual partners of 81 patients attending a sexually transmitted disease clinic who had chlamydial infections. Chlamydia trachomatis was isolated from 42% of the male partners and 62% of the female partners (p greater than 0.05). The number of infected partners was independent of the incidence of sexual intercourse and of the presenting symptoms. The use of condoms was the only contraceptive method that seemed to lower the risk of transmission of chlamydial infection. If diagnostic facilities for C trachomatis are not available, and sexual partners of patients with chlamydial infection are treated epidemiologically, a high proportion of both men and women may be treated unnecessarily.