Recent reports suggest that infection with Campylobacter jejuni, a common enteric pathogen, may cause Guillain-Barré syndrome (GBS) by triggering demyelination of peripheral nerves. GBS is preceded by an acute infectious illness (due to a variety of agents) in 50%-75% of cases; the onset of neurological symptoms is preceded by diarrhea in 10%-30% of cases. In the last decade, more than 20 published anecdotal reports and case series have described patients with C. jejuni infection documented 1-3 weeks before onset of GBS. Cultures of fecal samples obtained at the onset of neurological symptoms from patients with GBS have yielded C. jejuni in more than 25% of cases. A relatively rare serotype, Penner type O19, is overrepresented among isolates of C. jejuni from Japanese patients with GBS. Serological studies suggest that 20%-40% of patients with GBS have evidence of recent C. jejuni infection. In summary, infection with C. jejuni is a common antecedent to GBS and probably plays a role in initiating demyelination; although several pathogenic mechanisms are possible, none has been proven.