Do the long work hours required by many high-paying professions inhibit the entry of women? I investigate this question by studying a 2003 policy that capped the average workweek for medical residents at 80 hours. Using data on the universe of U.S. medical school graduates, I find that when a specialty reduces its weekly hours, more women enter the specialty, whereas there is little change in men’s entry. I provide evidence that the increase in women is due to changes in labor supply, rather than labor demand. At the residency program level, I document that baseline female representation predicts female entry after the reform. A back-of-the-envelope calculation suggests that the reallocation of women among medical specialties due to the hours reduction can close the physician gender wage gap by 11 percent.