The practice of maintaining large Institutional Review Boards (IRBs) raises the question whether membership that extends much beyond the minimal regulatory requirements is necessary. Anecdotal data suggest that chairs and reviewers assigned for each protocol may make the greatest contributions to discussions, but to date there have been no systematic data describing how frequently anyone other than an assigned reviewer or the chair participates in protocol discussions. If "ancillary" participants rarely speak, then it is difficult to argue that these participants play an important role in IRB deliberations. We use data from a unique observational study of IRBs at major academic medical centers to examine this question.