Sera of 85 patients with benign soft tissue lesions or sarcomas of soft tissues were investigated for a collagen metabolite, the aminoterminal propeptide of type III procollagen (PIIINP). Patients were divided into three groups: benign soft tissue lesions (n = 39), localised (n = 29) and metastatic (n = 18) soft tissue sarcomas (STS). Values of PIIINP above the reference range were found in 15%, 28% and 50% of the respective groups. The difference in the concentration of PIIINP was statistically significant between the benign lesions and the localised sarcomas; P = 0.05, and between the benign lesions and the metastatic sarcomas; P less than 0.001. In localised sarcomas there was a correlation between PIIINP and bone-involvement (r = 0.61, P = 0.002) and in metastatic disease between PIIINP and liver metastases (r = 0.77, P less than 0.001). In localised sarcomas the overall survival for patients with a value of PIIINP above the reference range was significantly poorer (P = 0.03) than for patients with values within the reference range, even after stratification for the histological malignancy grade of the tumours (P = 0.04).