Background: In Brazil, all studies involving human beings must be evaluated by an institutional review board (IRB) registered with the National Commission for Ethics in Research (CONEP), the entity responsible for coordinating all IRBs in the country. Methods: In 2007, a broad quantitative evaluation of Brazilian IRBs was carried out, followed by a qualitative component conducted using a semistructured interview technique during the last three months of 2008. Twenty IRBs situated in five geographical regions of the country and located within different social and institutional contexts were selected. Eighty interviews were conducted. Results: In general, the functioning of all the IRBs was similar. Problems were found related to the infrastructure provided for their work and noncompliance with the Resolution 196/96 recommendation that IRB members be given time off from their normal duties for their work with the IRB. The research protocols were usually evaluated by only one or two members. It was mentioned that investigators tended to be resistant to sending their projects for evaluation and to challenging the reports issued. In general, the IRBs did not follow up on the studies that they approved because they lacked the means to be able to do so. Conclusions: Although a large network of IRBs has been created in Brazil, many of these IRBs confront serious difficulties in attempting to comply with the regulations established by the National Health Council (NHC), which may affect the ability of some IRBs to fulfill their role.