This study compares 2 data sources to evaluate the quality of sexual health care provided to adolescents in Medicaid managed care: (a) Medicaid encounter data and (b) medical record data. Data from 1998 for 1112 adolescent enrollees came from 3 Seattle-area managed care organizations (MCOs): a group model health maintenance organization, an independent practice association, and a clinic network. Quality of care was tracked by estimating within-MCO chlamydia testing rates for sexually active female enrollees. Rates varied dramatically depending on which data source was used. Logistic regression models indicated substantially less difference between MCOs when analysis was based on data from the 2 sources combined than when based on either data source alone. Study results did not support the use of Medicaid encounter data as a sole data source for evaluating quality of adolescent sexual health care, despite the cost savings this would represent. However, encounter data, used as an adjunct to medical record review, may increase the reliability of quality evaluations.