This study examines practice mobility among young physicians, based on a large nationwide 1987 survey of physicians under age 40 years and in their second through fifth years of practice. Averaged across all physicians, there is a 10% to 12% chance of changing practices for each of the first 3 years, and about one in three physicians changes within the first 5 years. While the most common transition is from employee to self-employed practices, many practice changes are within employment type. Those starting in a self-employed solo practice are least likely to change practices, while those starting as HMO employees are most likely to change. Multinomial analysis of practice change choices reveals some specialty-specific differences in these choices, as well as the effects of experience, schooling, race, sex, debt, family constraints, and locational preferences. High debt is a factor in practice changes, but neither competition nor discrimination against minorities appear to play significant roles.