The prevalence of Chlamydia trachomatis infection in a population of women with no symptoms of sexually transmitted disease was investigated. These women, aged 35-55 years, participated in a screening program for cervical cancer. With the use of a direct immunofluorescence method, 109 out of 2,470 smears tested were positive for Chlamydia trachomatis, indicating an overall prevalence of 4.4%. No changes in prevalence were found when five-year cohorts of this group were analyzed, indicating that age-dependent changes or epidemiological factors do not result in a different (decreased) prevalence over the ages 35 to 55 years. The prevalence of Trichomonas vaginalis and fungi, as detected by cytological screening, was lower than that observed for Chlamydia trachomatis: 3.1 and 2.1%, respectively. Of the 109 smears positive for Chlamydia trachomatis, 90 showed cervical cells with reactive changes (out of 1,490 smears with PAP II), whereas no cytological changes were found in 15 cases (out of 884 smears with PAP I). Changes suggestive of mild or moderate dysplasia were found in only four cases (out of 93 smears with PAP III). The results indicate that Chlamydia trachomatis is associated with reactive changes of endocervical cells and raise serious questions about whether prevention of possible secondary effects such as infertility and pelvic inflammatory disease can be achieved by a combined screening program for cervical cancer and Chlamydia trachomatis.