We examine the role of gender in legal decision making by applying critical mass theory to the U.S. federal district courts. We analyze whether behavioral differences manifest themselves in the decision-making proclivities of male and female judges, contingent on the existence of a critical mass of female judges at a court point (i.e., each city in which a district court is located). Our results indicate that women jurists exhibit distinctive behavior in certain cases when there is a critical mass of women at a court point. These differences are most significant in criminal justice cases; modest differences between men and women are also identified in civil rights and liberties cases. Gender is not significant in labor and economic regulation cases. These findings suggest that the increasing presence of women on the federal bench could have substantial policy ramifications in the American polity.