In 10 patients with Guillain-Barré syndrome a preceeding cytomegalovirus infection could be demonstrated by virus-specific IgM antibodies that were present in high titers in 9 of the 10 patients in the first serum specimen. The IgM antibodies to cytomegalovirus were detected by a sensitive "double" indirect immunofluorescence technique. In most of our cases (8 of 10) the complement-fixing antibody titers had already reached high levels on admission into a hospital, and significant titer changes were not observed. Cytomegalovirus was isolated from the urine of five patients.