Endocervical cultures for Chlamydia trachomatis and Neisseria gonorrhoeae were taken from 492 women attending an outpatient clinic for sexually transmitted diseases (group I) and 560 women seeking legal abortion (group II). Possible risk factors for C trachomatis infection were evaluated by multivariate analysis. The prevalence rates for C trachomatis and N gonorrhoeae were 7.3% and 2.5% in group I and 9.4% and 0.4% in group II. From multivariate analysis it was found that age (p less than 0.01), number of sexual partners (p less than 0.01), abnormal vaginal discharge (p less than 0.01), and endocervical mucopus (p = 0.02) were independently associated with chlamydial infection in group I. In the abortion clinic age (p = 0.03) and endocervical mucopus (p = 0.03) were the only significant independent predictors of C trachomatis. In all women vaginal discharge was collected for Gram staining. A significant higher number of polymorphonuclear cells was seen in the smears of C trachomatis positive women (group I: p = 0.04; group II: p = 0.03). In group II there was also a significant association between C trachomatis and Gardnerella type bacterial flora (p = 0.02) and the presence of comma-shaped rods (p = 0.04). Screening for C trachomatis infection may help to decrease the incidence of (post-abortal) pelvic inflammatory disease. Because screening in abortion clinics is not always possible, decreasing the incidence of postabortal pelvic inflammatory disease could be achieved by using prophylactic antibiotics. Selective use of prophylaxis in high risk patients can minimise costs and the incidence of side effects.