A clinicoepidemiological study of 109 patients with Guillain-Barré syndrome who were admitted to the four major teaching hospitals in Perth between January 1, 1980 and December 31, 1985 was conducted through the Hospital Morbidity Data System. The annual incidence rate of Guillain-Barré syndrome was 1.35 cases per 100,000 population. The age-adjusted incidence rates were 1.49 cases per 100,000 men and 1.20 cases per 100,000 women. A minor peak in the sex-adjusted incidence rate was present in young adult life with a larger peak in later life. Twenty-two (20%) patients presented in the five-month period from July 1984 to November 1984 (P less than 0.05) but a common infectious agent or geographical area of residence was not discovered during this period. Immune function had been affected potentially before the onset of Guillain-Barré syndrome in 12 (11%) patients, which raises further speculation as to the role of immunological mechanisms in the pathogenesis of Guillain-Barré syndrome. In order to enhance our understanding of this disorder, it is recommended that an assessment of immune function be considered in cases of Guillain-Barré syndrome and also chronic inflammatory demyelinating polyneuropathy.