An ongoing surveillance program was intensified during the 1979-1980 and the 1980-1981 influenza seasons to determine whether an increased risk of acquiring Guillain-Barré syndrome (GBS) within eight weeks after influenza vaccination existed for adults in the United States who received influenza vaccine, when compared with adult who had not been vaccinated recently. Five hundred twenty-eight cases of GBS with onset between Sept 1 and March 31, including seven following recent vaccination, were reported by participating neurologists in 1979-1980; 459 cases, including 12 following recent vaccination, were reported in 1980-1981. The relative risk of acquiring GBS following influenza vaccination--0.6 in 1979-1980 and 1.4 in 1980-1981--was not significantly different from 1.0 in either season. These results suggest that there was no increased risk of acquiring GBS associated with the influenza vaccines administered during these seasons and that the causative "trigger agent" in the A/New Jersey (swine) influenza vaccine administered in 1976 has not been present in subsequent influenza vaccine preparations.