Context: An unexplained increase in the risk of Guil- lain- Barré syndrome (GBS) occurred among recipients of the swine influenza vaccine in 1976- 1977. Guillain- Barré syndrome remains the most frequent neurological condition reported after influenza vaccination to the Vaccine Adverse Events Reporting System (VAERS) since its inception in 1990. Objective: To evaluate trends of reports to VAERS of GBS following influenza vaccination in adults. Design, Setting, and Participants: VAERS is the US national spontaneous reporting system for adverse events following vaccination. Reports of GBS in persons 18 years or older following influenza vaccination were evaluated for each influenza season from July 1, 1990, through June 30, 2003. The number of people vaccinated was estimated from the National Health Interview Survey and US census data. Beginning in 1994, active follow- up was conducted to verify GBS diagnosis and obtain other clinical details. Main Outcome Measure: Reporting rates of GBS following influenza vaccination over time. Results: From July 1990 through June 2003, VAERS received 501 reports of GBS following influenza vaccination in adults. The median onset interval (13 days) was longer than that of non- GBS reports of adverse events after influenza vaccine (1 day) (P<.001). The annual reporting rate decreased 4- fold from a high of 0.17 per 100 000 vaccinees in 1993- 1994 to 0.04 in 2002- 2003 (P < .001). A GBS diagnosis was confirmed in 82% of reports. Preceding illness within 4 weeks of vaccination was identified in 24% of reported cases. Conclusions: From 1990 to 2003, VAERS reporting rates of GBS after influenza vaccination decreased. The long onset interval and low prevalence of other preexisting illnesses are consistent with a possible causal association between GBS and influenza vaccine. These findings require additional research, which can lead to a fuller understanding of the causes of GBS and its possible relationship with influenza vaccine. Context: An unexplained increase in the risk of Guilin-Barré syndrome (GBS) occurred among recipients of the swine influenza vaccine in 1976- 1977. Guillain-Barré syndrome remains the most frequent neurological condition reported after influenza vaccination to the Vaccine Adverse Events Reporting System (VAERS) since its inception in 1990. Objective: To evaluate trends of reports to VAERS of GBS following influenza vaccination in adults. Design, Setting, and Participants: VAERS is the US national spontaneous reporting system for adverse events following vaccination. Reports of GBS in persons 18 years or older the following influenza vaccination were as evaluated for each influenza season from July 1, 1990, through June 30, 2003. The number of people vaccinated was estimated from the National Health Interview Survey and US census data. Beginning in 1994 , active follow- up was conducted to verify GBS diagnosis and obtain other clinical details. Main Outcome Measure: Reporting rates of G BS: following influenza vaccination in adults. The median onset interval (13 days) was longer than that of non- GBS reports of adverse events after influenza vaccine (1 day) (P <.001). The annual reporting rate decreased 4- fold from a high of 0.17 per 100 000 vaccinees in 1993- 1994 to 0.04 in 2002- 2003 (P <.001). A GBS diagnosis was confirmed in 82% of reports. Preceding illness within 4 weeks of vaccination was identified in 24% of reported cases. Conclusions: From 1990 to 2003, VAERS reporting rates of GBS after influenza vaccination decreased. The long onset interval and low prevalence of other preexisting illnesses are consistent with a possible causal association between GBS and influenza vaccine. These findings require additional research, which can lead to a fuller understanding of the causes of GBS and its possible relationship with influenza vaccine.