Patients with severe iron overload may develop hepatic fibrosis due to iron toxicity. Unfortunately, the follow-up of the fibrogenic activity during treatment by histological examination of tissue biopsies carries potential side effects, and may therefore not be justified ethically. Recently, the serum concentration of procollagen type III peptide (S-PIIINP) has been shown to be a valid serum marker of the activity of collagen metabolism in conditions with hepatic fibrosis unrelated to iron overload. In order to evaluate the potential usefulness of this test in patients with fibrosis due to iron overload, we investigated the relationship between the PIIINP serum concentration and the size of iron overload in 18 patients with hereditary haemochromatosis (HH) and in 14 patients with transfusional iron overload. A close correlation was found between S-ferritin and S-PIIINP (r = 0.73, p < 0.0001). Follow-up of 6 patients during iron depletion treatment revealed a normalization of the serum aminotransferase concentration before normalization of S-PIIINP was found. This may indicate that excess iron directly induces an increase in fibrogenesis rather than the increased fibrogenesis is secondary to hepatocellular injury caused by iron excess. Thus, serial measurements S-PIIINP may be useful in follow-up of the fibrogenic process due to iron overload.