BACKGROUND: Patients at risk for generating high health care expenditures often receive fragmented, lowquality, inefficient health care. Guided Care is designed to provide proactive, coordinated, comprehensive care for such patients. OBJECTIVE: We hypothesized that Guided Care, compared to usual care, produces better functional health and quality of care, while reducing the use of expensive health services. DESIGN: 32-month, single-blind, matched-pair, cluster-randomized controlled trial of Guided Care, conducted in eight community-based primary care practices. PATIENTS: The "Hierarchical Condition Category" (HCC) predictive model was used to identify high-risk older patients who were insured by fee-for-service Medicare, a Medicare Advantage plan or Tricare. Patients with HCC scores in the highest quartile (at risk for generating high health care expenditures during the coming year) were eligible to participate. INTERVENTION: A registered nurse collaborated with two to five primary care physicians in providing eight services to participants: comprehensive assessment, evidence-based care planning, proactive monitoring, care coordination, transitional care, coaching for selfmanagement, caregiver support, and access to community-based services. MAIN MEASURES: Functional health was measured using the Short Form-36. Quality of care and health services utilization were measured using the Patient Assessment of Chronic Illness Care and health insurance claims, respectively. KEY RESULTS: Of the eligible patients, 904 (37.8 %) gave written consent to participate; of these, 477 (52.8 %) completed the final interview, and 848 (93.8 %) provided complete claims data. In intention-to-treat analyses, Guided Care did not significantly improve participants' functional health, but it was associated with significantly higher participant ratings of the quality of care (difference=0.27, 95 % CI=0.08-0.45) and 29 % lower use of home care (95 % CI=3-48 %). CONCLUSIONS: Guided Care improves high-risk older patients' ratings of the quality of their care, and it reduces their use of home care, but it does not appear to improve their functional health.