We measured the serum concentration of the aminoterminal propeptide of collagen type III (PIIIP) in 22 patients with acute viral hepatitis (19 hepatitis B, 3 hepatitis non-A, non-B). Nine patients showed persistent biochemical remission, 13 patients developed chronic active hepatitis (CAH); 6 of those underwent therapy with methylprednisolone and azathioprine. Thirteen patients with chronic persistent viral hepatitis (CPH) and 38 healthy individuals were also investigated. In the control group, PIIIP values were 9.5 +/- 2.25 ng/ml (chi +/- SD; range 4-14 ng/ml). All patients with acute hepatitis showed elevated PIIIP values (range 20-125 ng/ml). In the 9 patients with biochemical resolution, PIIIP normalized after a maximum of 6.5 months (range 7.5-14 ng/ml). In CAH, PIIIP was persistently elevated on the day of the diagnostic biopsy (range 15.6-35.7 ng/ml). In comparison, the patients with chronic persistent hepatitis showed a range of 5.0-15.4 ng/ml. Differences between controls and CAH and CPH/CAH were statistically highly significant (P less than 0.001). Treatment of patients with CAH by immunosuppression resulted in normal PIIIP values in 3 and persistently elevated values in 3. One additional patient had normal PIIIP after treatment with an increased dose of methylprednisolone of 16 mg p.d. Serum concentrations of PIIIP offer a non-invasive index for the development of chronic active hepatitis from acute viral hepatitis. This blood test may also be useful for monitoring immunosuppressive treatment in CAH.