Although viral load of hepatitis C virus (HCV) is a predictor of response to interferon therapy, little is known about its fluctuations. We assessed its fluctuations and their correlation with serum alanine aminotransferase (ALT) levels. Viral load was prospectively measured bimonthly for 22 months in 109 patients. In 40 patients, viral load changed more than five fold. Changes were transient and always returned to the baseline levels. ALT levels changed more than three fold in 30 patients. Changes of viral load accompanied simultaneous changes of ALT levels in only 7 of 40 patients with changes of viral load. Mean viral load in 22 months was significantly correlated with mean ALT levels inversely (r = 0.278, P = 0.0036). Mean viral load was significantly higher in 27 patients with persistently normal ALT levels (452.0 +/- 342.5pg/ml) than in 30 patients with changes of ALT levels (202.4 +/- 215.0pg/ml) (P = 0.0016) and than in 52 patients without changes of ALT levels (301.1 +/- 295.4pg/ml) (P = 0.0458). Inverse correlation of viral load with ALT levels suggests that viral load is in suppression by inflammatory activity. However, changes of ALT levels infrequently accompanied simultaneous changes of viral load and vice versa, as often seen in chronic hepatitis B virus infection.