Serum levels of the amino-terminal type III procollagen peptide (P-3-NP) have been used as an index of collagen synthesis. Systemic sclerosis (SS) is characterized by uncontrolled production of collagen of several types. This study aimed to explore the profile of P-3-NP in patients with SS and Raynaud's phenomenon, a common forerunner of the disease. Using a radioimmunoassay, the mean level for P-3-NP was found to be raised in SS compared with both the control (p less than 0.001) and Raynaud's groups (p less than 0.001). Analysis of serial samples from the patients with SS suggested that the P-3-NP level reflected changing clinical activity. Three groups emerged: a group with stable disease which showed a less than 20% change in P-3-NP level (mean 5.7%); a group with increasing activity which showed an increase of greater than 20% (mean 35.8%) and a group of decreasing activity which showed a decrease of greater than 20% (mean 33.6%). These data suggest that there is an increase in collagen metabolism in SS and that changes in P-3-NP levels may reflect the clinical course of the disease.