Unlike contact-tracing procedures for syphilis and gonorrhea, field follow-up to locate and treat patients with Chlamydia trachomatis infections has not been extensively applied in the United States. We implemented two studies to assess the efficiency and cost-effectiveness of using field follow-up for contact of two groups: patients with chlamydial infection detected as part of a screening program and women who were sexual partners of men with nongonococcal urethritis (NGU). Of the 142 patients with chlamydial infection who had not been treated empirically, 112 (79%) returned for treatment when a reminder system was used, as compared with a return rate of 97% (259/266) achieved by field follow-up (P less than 0.0001). Among the 678 men with NGU enrolled in a randomized trial of field follow-up vs. two self-referral methods, field follow-up yielded over three times as many partners returning to the clinic for treatment as did either of the other two methods (P less than 0.001). Analyses using the estimated costs of the intervention strategies and the medical costs associated with an untreated chlamydial infection showed that field follow-up by trained investigators proved to be not only the most efficient method for locating patients with chlamydial infection and/or patients who were at risk for it, but also the most cost-effective in terms of total health-care dollars spent.