All the 51 patients had history of plasma donation before onset of the illness. All had no evidences of recent infection with following viruses: HAV, HBV, CMV and EBV and were confirmed recently as hepatitis C by the Chiron C-100 EIA test. The incubation period was 35 to 90 days, with an average of 57.9 +/- 19.8 days. The ratio of icteric to anicteric was 1.0:1.55. Only a few cases had fever in the acute phase (13.7%). 36 cases showed acute onset of illness; 15 cases, however, had ALT elevation before the onset. 33 cases became chronic hepatitis after 1 year (64.7%), 14 out of the 33 cases had persistently abnormal ALT and 19 cases showed repeated elevation of ALT. The chronicity rate in 2 years was 38.6%. These results indicate that hepatitis C has the features of high chronicity rate and prolonged clinical course.