We have evaluated the cost-effectiveness of using cell culture to test for chlamydial infections in 9979 patients at a clinic for sexually transmitted diseases. From results of cultures, we have established prevalence data and, using decision-theory analysis, have calculated costs and probabilities of various outcomes. According to their histories and presenting signs and symptoms, patients were classified as at high or low risk for chlamydial infections. Empiric treatment of all patients attending the clinic was the most cost-effective strategy, followed by empiric treatment of high-risk women and culture-based treatment of low-risk women. Obtaining cultures for men at high and low risk was not cost-effective. If universal treatment is not provided, the most cost-effective strategy appears to be empiric therapy in patients at high risk for chlamydial infections and therapy based on diagnostic test results in women at low risk.