Waterborne non-A, non-B hepatitis (NANB) is responsible for outbreaks of hepatitis with a predilection for young adults. The disease is usually mild, except in pregnant women, who have a high case-fatality rate from fulminant hepatic failure. Diagnosis is largely based on the epidemiological findings of faecal contamination of drinking water and serological exclusion of hepatitis A and B virus infection. Histological features of liver biopsy specimens are characteristic and virus-like particles in the stool are aggregated by antibody present in acute and convalescent phase sera of the test subject. NANB is widespread in India and several countries of South-East Asia; it is increasingly recognised in Africa and may occur in Latin America. Control measures include provision of clean water supplies, safe disposal of human excreta, and sound personal and food hygiene practices. Passive immunisation with immunoglobulin derived from healthy donors resident in the countries affected by the disease may protect vulnerable groups.