Using longitudinal data on practicing obstetricians from 1998 to 2004 derived from Pennsylvania, Florida, and New York hospital discharge data, this study finds that rising malpractice premiums are associated with an increased rate of exit and a reduced rate of entry for obstetricians. In Pennsylvania and Florida, where premiums rose by an average of 20% per year for four years, the combined effect of more exits and fewer entries was associated with a decline in obstetrical labor supply of 80 physicians, or 5.3% of the supply of obstetricians.The medical malpractice environment has experienced several periods of great volatility over the past 30 years. The most recent crisis, which started in 2000, upset a decade of stability in premiums. It was characterized by average premium hikes of 15% per year, with particularly large premium increases in some geographic areas and in high-risk specialties such as neurosurgery and obstetrics (Rubin 2001;Guadagnino 2001), resulting in abrupt shifts in the cost structure of some physician practices. Although this raised concerns that the availability of medical services may be reduced in certain communities, little evidence exists to support these claims.This study uses rich new data to examine the direct relationship between the growth of malpractice premiums and the size and composition of the physician workforce specializing in obstetrics. The primary data sources for this project are statewide hospital discharge data sets from 1998 through 2004 from Pennsylvania, Florida, and New York. We first identify physicians practicing obstetrics in each state and each year by affirming whether hospital records indicate they performed deliveries in that year. Then, by linking physician information over time, we precisely determine the year when each physician stopped or started his or her obstetrical practice. We show that rising malpractice premiums increased the rate at which practicing obstetricians dropped their obstetrics practice and simultaneously reduced the rate of entry for new obstetricians.