Concomitant with a nationwide oral poliovirus vaccine (OPV) campaign in Finland in 1985, an unexpected rise in the number of patients hospitalized with the Guillain-Barré syndrome (GBS) occurred. An analysis based on hospital records covering a population of 1.17 million and 6 years revealed a significantly increased incidence of GBS coinciding with the OPV campaign. We carefully examined 10 cases with onset of GBS within 10 weeks after immunization. Only 4 patients could recall recent symptomatic infection, and we found no specific agent in these cases. Our study suggests that live-attenuated polioviruses may, like other infectious viruses, sometimes trigger the GBS.