Objective:To develop and test a diagnostic index for estimating the probability of cervical infection with either Chlamydia trachomatis or Neisseria gonorrhoeae. Design:Prospective, cross-sectional study in two phases: 1) to develop a diagnostic index based on independent predictors of cervical infection; 2) to test the index. Settings:A hospital-based clinic and a student health service. Patients:Development phase: 190 nonpregnant women seen in the gynecology clinic; testing phase: 588 women seen in the gynecology clinic (n=372) or the student health service (n=216). Interventions:Experienced clinicians recorded historical, physical, and microscopic findings on standard forms and tested women for chlamydial and gonococcal infections. Results:Three independent predictors of cervical infection were identified and weighted: age (two points if<20 years and one point if 20-29 years); a new sex partner or one suspected of having a genital infection (one point); purulent vaginal discharge (one point). In the testing groups, cervical infection was present in none of 62 women with no points, seven of 269 (3%) with one point, 14 of 188 (7%) with two points, and 19 of 69 (28%) with three or four points (p<0.001). The index estimated the probability of infection more accurately (p<0.01) than did clinicians, performed well in each site, and remained accurate when C. trachomatis and N. gonorrhoeae were considered separately. Conclusion:The diagnostic index accurately estimates the probability of cervical infection with either C. trachomatis or N. gonorrhoeae and may be useful in selecting women for definitive diagnostic testing. © 1990 Society of General Internal Medicine.