Objective: To evaluate chlamydia-screening policies, testing practices, and the proportion testing positive in response to the new Health Plan Employer Data and Information Set (HEDIS) chlamydia-screening performance measure in a large commercial health plan. Methods: We interviewed health plan specialty departmental chiefs to describe interventions used to increase chlamydia screening and examined electronic medical records of 15- to 26- year- old female patients 37,438 from 1998 to 1999 and 37,237 from 2000 to 2001- who were classified as sexually active by HEDIS specifications to estimate chlamydia testing and positive tests 2 years before and after the HEDIS measure introduction. Results: In January 2000, the obstetrics and gynecology department instituted a policy to collect chlamydia tests at the time of routine Pap tests on all females 26 years old or younger by placing chlamydia swabs next to Pap test collection materials. Other primary care departments provided screening recommendations and provider training. During 1998- 1999, 57% of eligible female patients seen by obstetrics and gynecology exclusively and 63% who were also seen by primary care were tested for chlamydia; in 2000- 2001 the proportions tested increased to 81% (P < .001) and 84% (P < .001). Proportions tested by other primary care specialists did not increase substantially: 30% in 1998- 1999 to 32% in 2000- 2001. The proportion of females testing positive remained high after testing rates increased: 8% during 1998- 1999 and 7% during 2000- 2001, and the number of newly diagnosed females increased 10% . Conclusion: After the obstetrics and gynecology department introduced a simple systems-level change in response to the HEDIS measure, the proportion of females chlamydia-tested and number of newly diagnosed females increased. Objective: To evaluate chlamydia-screening policies, testing practices, and the proportion testing positive in response to the new Health Plan Employer Data and Information Set (HEDIS) chlamydia-screening performance measure in a large commercial health plan. Methods: We interviewed health plan specialty departmental chiefs to describe interventions used to increase chlamydia screening and examined electronic medical records of 15- to 26-year-old female patients 37,438 from 1998 to 1999 and 37,237 from 2000 to 2001- who were classified as sexually active by HEDIS specifications to estimate Results: In January 2000, the obstetrics and gynecology department instituted a policy to collect chlamydia tests at the time of routine Pap tests on all females 26 years old or younger placed by chlamydia swabs next to Pap test collection materials. Other primary care departments provided screening recom mendations and provider training. Between 1998 and 1999, 57% of eligible female patients seen by obstetrics and gynecology exclusively and 63% who were also seen by primary care were tested for chlamydia; in 2000- 2001 the percentage of patients tested increased to 81% (P <.001) and 84% (P <.001). Proportions tested by other primary care specialists did not increase substantially: 30% in 1998- 1999 to 32% in 2000- 2001. The proportion of females testing positive remained high after testing rates increased: 8% during 1998- 1999 and 7% during 2000- 2001, and the number of newly diagnosed females increased 10%. Conclusion: After the obstetrics and gynecology department introduced a simple systems-level change in response to the HEDIS measure, the proportion of females chlamydia-tested and number of newly diagnosed females increased.