An epidemiologic study of Guillain-Barré syndrome in the well-defined population of San Joaquin County, California is presented. During the period 1972-1976, 18 cases occurred, yielding an average annual incidence rate of 1.23 cases/100,000 population/yr, age-adjusted to the 1950 Olmsted County, Minnesota population. The results of this study are similar to those of the only other epidemiologic study of Guillain-Barré syndrome in the continental United States, which was done in Olmsted County, Minnesota. No significant sex predilection, seasonal variation, or common antecedent events were found. © 1979.