Hepatitis non-A-non-B is a nosologically homogeneous group of diseases probably caused by different infectious agents. Laboratory tests specific for these agents are not yet available. Epidemiologically one has to distinguish occurence of the disease after transfusion and sporadically occuring disease. In addition, epidemics have been reported, which seem to be similar to hepatitis A virus infection in regard to epidemiological and clinical pattern. Hepatitis non-A-non-B makes up for about 25% of all cases with hepatitis caused by virus, sporadically occuring cases predominate. In our own group of patients only 10% occured after transfusion. Hepatitis non-A-non-B occurs in adults, about equally in males and females; there are no saisonal influences to be observed. Blood transfusion, substitution of coalgulation factors, parenteral drug abuse and hemodialysis represent definite high risk situations. Nosocomial infections in professional people (doctors, nurses) and infections of sexual partners occure more rarely as is the case in hepatitis B. Perinatal transmission of the infectious agents of hepatitis non-A-non-B still has to be clarified.