A nationwide study of 24 medical and surgical specialties has been conducted by the University of Southern California School of Medicine, Division of Research in Medical Education. This article is the first in a series reporting findings for general internal medicine and 10 subspecialties of internal medicine. Populations for these 11 specialties are defined and enumerated, and the specialties are compared in terms of demographic and geographic distribution. Practice comparisons are presented based on characteristics such as workload, allocation of professional time, location of encounters with patients, distribution of primary problem diagnoses, and projections of annual patient encounters. Forthcoming specialty-specific articles will present highly detailed information for general internal medicine and for the subspecialties of cardiology, gastroenterology, pulmonary disease, allergy, hematology, endocrinology, nephrology, medical oncology, rheumatology, and infectious diseases.