Background: Since CT testing for females is a new Health Plan Employer Data and Information Set (HEDIS) measure to assess MCO quality of care, we determined the proportion of females enrolled in a large closed panel vertically integrated MCO serving a demographically diverse population who were tested for CT at least once during 1998 and resulting CT prevalences.Methods: A data base with MCO members ⩾ 12 yrs old tested with CT DNA probes (GenProbe) from 1/1/98–12/30/98 was examined. Only females 12–24 yrs old and enrolled at least 11 continuous months in 1998 were included in the analysis.Results: In 1998, 5425 (13.1%) of 41,566 females 12–24 yrs were CT tested and 849 (2.0%) tested positive at least once. Proportion of females tested and proportion of females testing positive at least once varied by age and clinic location. Among 11,562 12–14 yr females, 277 (2.4%) were CT tested and 48 (0.4%) tested positive; among 18,155 15–19 yr females, 2,424 (13.4%) were tested and 572 (3.2%) tested positive; and among 11,849 20–24 yr females, 2,724 (23.0%) were tested and 229 (1.9%) tested positive. The proportion 12–24 yr females tested was 579/3,945 (14.7%) with 92/3,945 (2.3%) testing positive in the Baltimore area; in Washington, DC/Maryland area 2,999/21,085 (14.2%) were tested with 601/21,085 (2.9%) testing positive; in Northern Virginia 1,847/16,536 (11.2%) were tested with 156/16,536 (0.9%) testing positive.Proportion of females with at least one positive test among those females who were tested for CT also varied by age and clinic location. Among 277 of 12–14 yr females tested 48 (17.3%) tested positive at least once; among 2,424 of 15–19 yr females tested, 572 (23.6%) tested positive at least once; and among 2,724 of 20–24 yr females tested, 229 (8.4%) tested positive at least once. The proportion of females tested with at least one positive test among 12–24 yr females was 92/579 (15.9%) in the Baltimore area, 601/2,999 (20.0%) in Washington DC/Maryland area, and 156/1,847 (8.4%) in Northern Virginia.Conclusion: CT testing of adolescent females 12–19 yrs revealed a high proportion of positive tests. The lower proportion of positive tests in young adults 20–24 yrs may be due to differences in prevalence and increased number of tests performed. CT testing practices and rates varied by location. Therefore, in a large MCO, a uniform system wide approach in identifying and screening sexually active adolescent females may identify a large reservoir or asymptomatic infection.