We examined the value of serum procollagen III amino propeptide (PIIIP) for predicting the histological progression of primary biliary cirrhosis (PBC). Serial PIIIP measurements were obtained for nine patients with histologically progressive PBC and nine patients with histologically stable early disease, assessed by repeated liver biopsies and followed for up to 13 years. The means of the follow-up PIIIP concentrations were elevated in 39% of the cases; moreover, PIIIP levels were elevated at least once during follow-up in 72% of the cases. Mean follow-up PIIIP concentrations did not differ significantly between progressive and non-progressive patients. In addition, in the progressive group, histological progression was not reflected by PIIIP levels. No difference was found between the serum PIIIP levels corresponding to the histological stages I, II and III. The individual coefficients of the correlation between serum PIIIP and biochemical variables (bilirubin, alkaline phosphatase, ASAT, albumin) and histology showed a wide distribution without a consistent trend towards positive or negative. Treatment with cyclosporin A or cyclosporin A combined with prednisone did not influence serum PIIIP levels. Treatment with penicillamine combined with prednisone, however, resulted in a significant decrease in PIIIP concentrations (p less than 0.05). We conclude that serum PIIIP measurements are of no value for predicting the histological progression of PBC.