BACKGROUND Although infection with Campylobacter jejuni is recognized as a common antecedent of the Guillain-Barré syndrome, the clinical and epidemiologic features of this association are not well understood. METHODS We performed a prospective case-control study in a cohort of patients with Guillain-Barré syndrome (96 patients) or Miller Fisher syndrome (7 patients) who were admitted to hospitals throughout England and Wales between November 1992 and April 1994. Bacteriologic and serologic techniques were used to diagnose preceding C. jejuni infection. RESULTS There was evidence of recent C. jejuni infection in 26 percent of the patients with Guillain-Barré or Miller Fisher syndrome, as compared with 2 percent of household controls and 1 percent of age-matched hospital controls (P < 0.001). Of the 27 patients with C. jejuni infection, 19 (70 percent) reported having had a diarrheal illness within 12 weeks before the onset of the neurologic illness. No specific serotypes were associated with Guillain-Barré syndrome. C. jejuni infection was slightly more common in men (P = 0.14) and was more likely to be associated with a pure motor syndrome and a slower recovery (P = 0.03). The patients with preceding C. jejuni infection were more likely to have acute axonal neuropathy or axonal degeneration in association with acute inflammatory demyelinating polyradiculoneuropathy, and they had greater disability after one year (P = 0.02). C. jejuni infection was significantly associated with a poor outcome even after correction for other factors associated with a poor prognosis. CONCLUSIONS Infection with C. jejuni often precedes the Guillain-Barré syndrome and is associated with axonal degeneration, slow recovery, and severe residual disability.