The reliability of eight self-reported risk factors as criteria for screening women for Chlamydia trachomatis was evaluated in four family planning clinics in New York State that serve diverse populations. In all, 8,920 women were screened in these clinics; the rates of infection ranged from 2% to 7%. Results of multivariate analyses showed that age was the most important predictor of chlamydial infection in the three clinics where prevalence was 4% or higher; women aged 20-24 were 3-4 times as likely as older women to be infected, and those aged 13-19 were 4-6 times as likely. In these three clinics, screening all women aged 26 or younger (62-80% of the clinic population) would identify about 90% of infected women; in the clinic with the lowest prevalence rate, age was not a reliable criterion. The prevalence of self-reported risk factors varied by clinic, and these factors generally were not reliable indicators of infection. Using the presence of at least one self-reported risk factor as a screening criterion, 80-87% of clinic clients would be screened, and about 90% of infected women would be identified. The presence of clinical signs of chlamydial infection does not increase the reliability of age as a screening criterion.