Results. Of the 2607 eligible women, 1009 were randomly assigned to screening and 1598 to usual care. A total of 645 women in the screening group (64 percent) were tested for chlamydia; 7 percent tested positive and were treated. At the end of the follow-up period, there had been 9 verified cases of pelvic inflammatory disease among the women in the screening group and 33 cases among the women receiving usual care (relative risk, 0.44; 95 percent confidence interval, 0.20 to 0.90). We found similar results when we used logistic-regression analysis to control for potentially confounding variables.