A cluster of acute non-A, non-B hepatitis comprising 12 blood donors was diagnosed in a plasmapheresis unit. Nine cases were followed-up for 2-5.5 years and seven out of them progressed to chronicity, as judged by biochemical abnormalities. In six, liver biopsy was performed 1 year after the acute disease revealing chronic active hepatitis in two, chronic persistent hepatitis in two, chronic lobular hepatitis in one and normal liver in one. Repeated biopsies showed progression to cirrhosis in one case of chronic active hepatitis, and resolution of the disease in another one, while in the remaining patients liver morphology remained unchanged. Circumstantial epidemiologic evidence suggests a single agent being the cause of the outbreak, which resulted in a broad spectrum of liver disease.