In 1992, the Ad Hoc Task Force on Definition of the Medical Home of the American Academy of Pediatrics (AAP) (Pediatrics 1992; 90: 774) defined the recommended elements and characteristics of an appropriate medical home for children. To date, there have been few attempts to measure the health care significance of this pediatric medical home concept. We developed an algorithm based upon the AAP's definition in order to classify medical home status and used it in the analysis of a comprehensive statewide population-based survey (Child Quest 2000), designed to measure children's (age 6-36 mos) access to care and its correlates. Survey response variables used in developing the algorithm related to receipt of preventive services, continuity of well and sick care, and telephone access to medical advice. Data for this study are based upon an initial sample of 1000 children born 1991-93 and randomly drawn from Delaware's birth registry. To focus on high-risk children, indigent families were oversampled by one-third based on mother's health insurance at delivery. Of the 750 determined to have received the mailed survey, 502 responded (66.9%). The medical home algorithm separated the children into three groups: those with a medical home (69%), those without, the medically homeless (19%), and those with indeterminate responses(12%). Analyses of the health care implications of medical homelessness demonstrated a 2.1 fold increase (p<0.05) in ER utilization for non-urgent care regardless of insurance status; a 2.9 fold increase (p<0.05) in the history of inadequate prenatal care; and a 1.5 fold increase (p<0.05) in the risk of delayed immunization at age 3 months. These findings support the recommendations of the AAP's Ad Hoc Task Force on Definition of the Medical Home. The data suggest that the operationalized definition of medical home presented here should be a valuable tool in the design and assessment of child health care delivery models, particularly those targeting high-risk children and families.