Information concerning their clinical illness was obtained by self-administered questionnaire from 528 patients who suffered from epidemic polyarthritis in Australia during 1980 and 1981. The clear and uniform clinical picture which has emerged from this study should facilitate the diagnosis of this disease. The illness begins suddenly with the onset of acute aching in the muscles and joints, followed by a maculopapular rash in between 40% and 78% of patients, and extensive polyarthritis. This affects chiefly the ankles, fingers, knees and wrists, usually serially, but no joint is spared. Estimates of incubation time suggest that it ranges from three to 21 days (mean, nine days). Women, especially housewives, are the group most frequently affected. Exposure to mosquitoes is an important predisposing factor; the implications of this for prevention and future research are considered. The mechanism of the survival of the virus in nature remains poorly understood.