It is currently believed that non-A, non-B hepatitis represents several different viral infections. Although the condition is clearly distinct from common cases of hepatitis A and B, new forms of non-A, non-B hepatitis have recently been found that share some characteristics with these diseases. Reliable serologic assays do not exist, but animal transmission and electron microscopic studies indicate the presence of more than one viral agent. The great risk of chronicity following post-transfusion non-A, non-B hepatitis contrasts with the infrequent chronicity of sporadic non-A, non-B disease. Chronic non-A, non-B hepatitis is usually clinically inapparent with mild biochemical abnormalities; nevertheless, it slowly but relentlessly progresses to cirrhosis and portal hypertension.