Gram-negative bacteria of the genus Campylobacter and of related genera frequently colonize the gastrointestinal tracts of humans, other mammals, and birds. One organism, Campylobacter jejuni, has been recognized as an important human pathogen, usually causing a diarrheal illness. Infection is common throughout the world, but clinical and epidemiologic features differ in developed and developing countries. The high incidence of C. jejuni infections and their propensity to invade tissue and to induce inflammation are compatible with a role in the causation of Guillain-Barré syndrome.