The serum concentrations of a metabolite produced during synthesis type III collagen, type III procollagen peptide (PCP), were measured in patients with cryptogenic fibrosing alveolitis, and the results were related to the course of the disease and its response to treatment. The mean PCP concentration was significantly higher for patients untreated at the time of sampling than for an age and sex matched control group (p less than 0.001). Seventeen of these patients subsequently started treatment and their response to treatment was assessed by measurement of changes in the transfer coefficient (KCO) and forced vital capacity (FVC) at six months and one year later. Those patients showing a response to treatment, indicated by an increase in the percentage predicted KCO or FVC of more than 10% one year after starting treatment, had a significantly higher mean PCP concentration (p less than 0.01 and p less than 0.001 respectively) than patients with stable or deteriorating physiological indices. When the PCP concentrations of these patients were compared with repeat measurements three to six months after the commencement of treatment 14 out of 17 showed a decrease in PCP levels. This percentage change in PCP concentrations correlated significantly with the change in the percentage predicted KCO and FVC at 12 months after the commencement of treatment (p less than 0.01). These results suggest that measurement of PCP concentrations in patients with cryptogenic fibrosing alveolitis may be useful in the assessment of response to treatment.