From June to December 1985 273 cases of hepatitis and jaundice were diagnosed in Maun, northern Botswana. It was known before the outbreak that most adults were immune to hepatitis A virus, most had markers indicating past infection with hepatitis B virus (HBV), there was a mean prevalence of 13.6% (57/418) HBsAg carriers, and a proportion of people had antibodies to hepatitis delta virus (HDV). There was evidence that faecal contamination of water supplies preceded the outbreak; the epidemic curve suggested that there was a major common source of infection; the disease appeared to have affected 1-2% of the population; 90.3% (214/237) patients for whom information is available were aged 20 years or older; the disease was generally mild and affected pregnant women most severely. 49 patients were admitted to hospital and at least 4/273 died. There were prevalences of 47% (28/60) HBsAg and 69% (37/54) anti-HDV reactors among patients bled 1-43 d after the onset of illness. The main features of the outbreak conformed to published descriptions of water-borne epidemic non-A non-B hepatitis and it is postulated that the disease was most severe in patients with active HBV infection and in those with HDV superinfection, so that such patients were disproportionately represented amongst those seeking medical attention.