At the June 2001 meeting of the Advisory Committee for Immunization Practices (ACIP), seven cases of multiple organ system failure (MOSF) in recipients of 17D-derived yellow fever (YF) vaccine were presented. In response, an ACIP working group was formed to review the cases, assess the risk for serious adverse events following YF vaccination, and consider revision of the 1990 YF vaccination recommendations. This notice summarizes these cases and describes an enhanced surveillance program designed to refine risk estimates and improve histopathologic documentation of MOSF potentially associated with YF vaccination.