In what proportion of a patient's total health care is the primary physician involved? By means of calendar diaries and telephone interviews, 211 primary care patients from community practices of the Dartmouth Primary Care Cooperative Information Project were followed prospectively for one year. We found that a substantial proportion of care was managed (that is, either actually provided or coordinated in advance) by the patient's primary physician. Specifically, criteria for a primary physician's role in management were met by 75% of 1379 ambulatory visits to physicians, 33% of 786 visits to nonphysician health care providers, 81% of 26 nonemergency hospitalizations, and 78% of 2769 prescriptions. Primary physicians in these settings appear to function as case managers even when they are not participating in formal managed-care systems.