OBJECTIVE/GOAL: To understand the potential impact of assumptions about the natural history of untreated Chlamydia trachomatis on the cost-effectiveness of screening strategies. STUDY DESIGN: Using a previously developed state-transition model, we explored how alternative assumptions about the natural history of disease following infection affect the estimated cost-effectiveness of screening for U.S. women. The analysis was conducted from a modified societal perspective and incorporated a lifetime analytic horizon. RESULTS: Different natural history assumptions affect cost-effectiveness outcomes. Assumptions about the combined risk of persistent and repeat infections have the greatest impact on the composition of optimal screening strategies, whereas assumptions about the risk of pelvic inflammatory disease most greatly influenced the magnitude of incremental cost-effectiveness ratios. CONCLUSIONS: Priorities for future C trachomatis research should include better estimates of the risk of pelvic inflammatory disease, persistence, and repeat infection. Better delineation of these variables will permit improved evaluation of potential screening activities. Copyright © 2006, American Sexually Transmitted Diseases Association All rights reserved.