This paper estimates the effects of initial committee seniority on the career histories of Democratic members of the House of Representatives from 1949 to 2006. When more than one freshman representative is assigned to a committee, positions in the seniority queue are established by lottery. Randomization ensures that queue positions are uncorrelated in expectation with other legislator characteristics within these groups. This natural experiment allows us to estimate the causal effect of seniority on a variety of career outcomes. Lower-ranked committee members are less likely to serve as subcommittee chairs on their initial committee; are more likely to transfer to other committees; and have fewer sponsored bills passed in the jurisdiction of their initial committee. On the other hand, there is little evidence that the seniority randomization has a net effect on reelection outcomes or noncommittee bills passed.