BACKGROUND: A large and increasing proportion of health care costs are spent caring for a small segment of medically and socially complex patients. To date, it has been difficult to identify which patients are best served by intensive care management. OBJECTIVE: To characterize factors that best identify which complex patients are most suited for intensive care management. DESIGN: We conducted a mixed-methods study involving 35 care managers (CMs; 10 licensed social workers and 25 registered nurses) working in intensive care management programs within Kaiser Permanente Northern California (KPNC) outpatient medical centers. We asked CMs to review a randomly selected list of up to 50 patients referred to them in the prior year and to categorize each patient as either (1) "good candidates" for care management, (2) "not needing" intensive care management, or (3) "needing more" than traditional care management could provide. We then conducted semi-structured interviews to understand how CMs separated patients into these three groups. RESULTS: CMs assigned 1178 patients into the 3 referral categories. Less than two thirds (62%, n = 736) of referred patients were considered good candidates, with 18% (n = 216) categorized as not needing care management and 19% (n = 226) as needing more. Compared to the other two categories, good candidates were older (76.2 years vs. 73.2 for not needing and 69.8 for needing more, p < 0.001), prescribed more medications (p = 0.02) and had more prior year outpatient visits (p = 0.04), while the number of prior year hospital and emergency room admissions were greater than not needing but less than needing more (p < 0.001). A logistic regression model using available electronic record data predicted good candidate designation with a c statistic of 0.75. Several qualitative themes emerged that helped define appropriateness for referral, including availability of social support, patient motivation, non-medical transitions, recent trajectory of medical condition, and psychiatric or substance use issues. CONCLUSION: Many apparently complex patients are not good candidates for intensive care management. Current electronic medical records do not capture several of the most salient characteristics that determine appropriateness for care management. Our findings suggest that systematic collection of social support, patient motivation, and recent non-medically related life change information may help identify which complex patients are most likely to benefit from care management.