During late January 2003, the Michigan Department of Community Health (MDCH) received reports of severe unexplained illnesses and deaths in children and young adults aged <21 years residing in Michigan. Subsequently, two of the deaths were found to be associated with influenza, including one with neurologic complications. To identify cases of severe influenza in otherwise healthy children and young adults aged <21 years, MDCH conducted enhanced surveillance for influenza-associated illness. This report summarizes the findings of this ongoing investigation, which indicate the need to better define the frequency of serious complications from influenza in healthy children and to incorporate such findings into evaluations of current vaccine recommendations for children.