During the past 3 years, 30 sexual partners including 18 married couples and 12 extramarital sexual pairs whose male partners were diagnosed as having non-gonococcal urethritis were examined for Chlamydia trachomatis infection. Twenty-three of the 30 couples (76.7%) had identical results either positive or negative for Chlamydia trachomatis infection. All 3 male partners of the 3 pairs who had the non-identical results for Chlamydia trachomatis infection, male negative and female positive, had history of urethritis or prostatitis. Fourteen of the 17 female partners (82.4%) who were positive for Chlamydia trachomatis, had no subjective complaints. The above findings suggest the necessity of treating the female sexual partners of the non-gonococcal urethritis patients irrespective of their symptom.