OBJECTIVE: The Medical Home is a clinical practice concept that sets new standards for pediatric primary care. This study describes the development and validation of a tool to measure the Medical Home. METHODOLOGY: The Medical Home Index (MHI) consists of 25 themes arranged among 6 domains of primary care office activity. A national panel of experts on the Medical Home reviewed a prototype of the instrument. Pediatric primary care offices completed the MHI and participated in 90-minute on-site interviews with two of the authors (J.W.M., K.S.). The study examined interrater reliability between the 2 project staff and between the practices and project staff and the internal consistency of MHI domains and themes. RESULTS: On a 100-point scale, 43 practices demonstrated a range of MHI summary scores from 18.9 to 75.4, with a mean of 43.9. Kappa coefficients of interrater reliability between two Center for Medical Home Improvement project staff were above.50 for all 25 themes. Kappa scores comparing each staff member and the practice sites' self-assessment found 80% of the themes at.65 or better for one interviewer (J.W.M.) and 60% of the themes at.65 or better for the second interviewer (K.S.). Intraclass correlation coefficients between the summary scores of the interviewers were.98 and between the scores of the two interviewers and the scores of the practices was.97. The internal consistency reliability standardized alpha coefficients across the 6 domains of the MHI ranged from.81 to.91, and the overall standardized alpha coefficient was.96. CONCLUSIONS: In the sample of practices studied, the MHI was an internally consistent instrument with acceptable reliability and validity for pediatric primary care practices to assess their implementation of the Medical Home concept. Further work is needed to study its correlation with other process and outcome measures and its performance in a wider range of practices.