In the Scandinavian countries today, Chlamydia trachomatis seems to be the most important cause of sexually transmitted diseases. Although C. trachomatis is most prevalent in symptomatic women and in persons visiting venereal disease clinics, the asymptomatic carrier rate, particularly in young women, appears to be surprisingly high. In addition to young age and level of sexual activity, the use of oral contraceptives and the presence of cervical ectopy are important risk factors for chlamydial infection. Chlamydial cervicitis often is characterized by a hypertrophic follicular appearance, with severe edema, erythema, friability, and endocervical mucopurulent discharge. On colposcopy, an atypical transformation zone is frequently observed. Papanicolaou smears taken from C. trachomatis culture-positive patients often reveal benign atypias and dyskaryotic changes. C. trachomatis, therefore, being a venereally transmitted intracellular organism, may well play a role in the development of cervical intraepithelial neoplasia (CIN). Future studies on patients with cytological atypias, therefore, also should include tests for C. trachomatis. Prospective studies are needed to determine the natural history and outcome of C. trachomatis-associated atypias and their behavior after adequate therapy. A large proportion of C. trachomatis-infected women also are culture-positive from the urethra. The demonstration that C. trachomatis is a common causative agent of acute urethral syndrome (also called abacterial urethritis, dysuria-pyuria syndrome) in women has been a valuable contribution. The diagnosis of pyuria in a patient with abacterial dysuria is of critical importance, since it suggests a chlamydial etiology and thus is a potentially treatable condition.