The Baltimore City Primary Care Study examined the role of the urban office-based physician in the delivery of primary care. During the course of the study, questionnaires were completed by over 90 per cent of licensed physicians in the city, and data were collected from a sample of 16,000 patients. The study developed and assessed three approaches for the measurement of primary care. An empirical method was based upon information regarding the following characteristics of each visit: first contact visit, referral visit, specialized care visit, or principal care visit. A normative method was based on an assessment of the degree to which the practice provided care that was comprehensive, longitudinal, accessible, and family-centered. The third method used physicians' judgments as to the proportion of patients for whom they maintained ongoing responsibility for general medical care. All three methods produced the same categorization of different specialties as providing either primary, intermediate, or specialty care. As a group, primary care physicians included general and family physicians, pediatricians, and general internists. General surgeons and obstetrician/gynecologists have characteristics of both primary care and specialist care. All other physician groups have characteristics of specialists. Any of these methods or a combination of methods can be used to assess the extent to which the practice of a particular physician or group of physicians provides primary care.