With recent advances in the development of detection methods, the number of STD (Sex transmitted disease) cases detected is greater than ever. Among many STD, Chlamydia trachomatis (= CT), a microbe that has been given a great deal of attention in the urologic field as a cause of nonspecific urethritis and has a characteristic life cycle, has been gradually proved to exist commonly and to become a cause of various obstetrical and gynecological diseases. However, as it is clinically still unknown which symptoms are actually influenced by CT, we studied infection with CT in 706 cases, consists of a group of patients with some symptoms and an asymptomatic group mainly consisting of pregnant women. As a result, the positive rate was found to be 12.3% (87 cases); 10.8% (21 out of 194) in asymptomatic pregnant women, 11.0% (13 out of 118) in patients with cervicitis, and 14.9% (30 out of 202) in patients with adnexitis including PID. As to age, the positive rate was highest, 23.1%, in patients under 20 years old. In fact, 1) CT positive pregnant women are not rare, and 2) CT was detected in high frequency in the young generation under 20 years old; and in these young women, the possibility of transmission of the disease to the infant in the birth canal should be considered in connection with future pregnancy and delivery. Tubal sterility may also result. We concluded that it was necessary to establish a satisfactory examination system.