Because a cluster of cases of Guillain-Barré syndrome followed a mass vaccination program using oral polio vaccine, a retrospective epidemiologic survey was carried out in southern California to examine the relationship between oral polio vaccine and Guillain-Barré syndrome. We also examined the yearly pattern of Guillain-Barré syndrome and compared rates of illness in different ethnic populations. The mean annual incidence of Guillain-Barré syndrome was 0.60 (95% confidence intervals 0.48 to 0.73) per 100,000 children under 15 years of age. There was no difference in incidence between boys and girls or between ethnic populations. The incidence was significantly higher in 2-year-old children than in any other age group (p < 0.05). The onset of Guillain-Barré syndrome was preceded by an infection in 71 (76%) of 93 children. In five children (5%) a vaccination was given within 8 weeks before illness onset. Only two children received the oral polio vaccine; the duration from vaccine administration to the onset of Guillain-Barré syndrome was 41 and 52 days, whereas the mean incubation time associated with infectious illnesses was 10.3 days. Strong evidence against a causal relationship between oral polio vaccine administration and Guillain-Barré syndrome is provided by the failure to find a correlation between the usual age of immunization with oral polio vaccine and the incidence of Guillain-Barré syndrome by age and the failure to find any children with onset of Guillain-Barré syndrome within 1 month of immunization with oral polio vaccine.