Chronological measurements of various HCV markers were conducted to clarify the course and prognosis of acute hepatitis C. Among 49 patients with acute non-A, non-B hepatitis, 32 (65.3%) were diagnosed as having acute hepatitis C by these markers. Twenty-four (82.8%) of 29 patients with posttransfusion hepatitis were type C, while only eight (40.0%) of 20 patients with sporadic hepatitis were type C. Patients were also divided into those who returned to normal within one year based on changes in s-ALT levels and unresolved cases. Anti-HCV was present in 11 (44.4%) of 25 resolved cases and in 21 (87.1%) of 24 unresolved cases. Only one case was continuously positive for HCV-RNA although s-ALT levels returned to normal. In addition, quantitative determinations were conducted on those positive for anti-HCVs. Anti-second generation tested positive in all HCV-RNA-positive cases and positive rates were highest from the onset of hepatitis. In unresolved patients with continuous HCV infection, anti-core increased and titers at 12 months were 10 units or more in all cases. On the other hand, in eight of 10 resolved cases, titers declined gradually after initial seroconversion and titers were 10 units or less at 12 months. From these results, second generation anti-HCV was considered most useful in early diagnosis of acute hepatitis C and anti-core titer was considered most useful in predicting prognosis of acute hepatitis C.