Cytomegalovirus was isolated from the PBBC from a patient with acute idiopathic polyneuritis. Viremia occurred during the early phase of the neurologic disease, preceding viruria, and was accompanied by specific CMV-IgM antibodies, which indicates a recently acquired infection. Complement fixing antibodies to CMV remained stable throughout the illness at a relatively low titer. Previously, CMV has been associated with acute idiopathic polyneuritis on the indirect evidence of rising or elevated titers of complement fixing antibodies or isolation of virus from urine or saliva or both. Our findings provide direct evidence for the causative role of CMV in acute idiopathic polyneuritis.