We compared the type I and III collagen amounts and cross-linked telopeptides at the rupture site and two other sites of the same tendon. Tendon samples of ten individuals with total Achilles tendon rupture and six healthy cadavers were collected. The newly synthesized type I and III procollagens were assessed by extracting the soluble propeptides PINP, PICP and PIIINP. The insoluble matrix was solubilized by heat denaturation and trypsin digestion. Hydroxyproline, the cross-linked telopeptide structures of type I (ICTP and SP 4) and III collagens (IIINTP) and the degradation product of type III collagen (tryptic PIIINP) were measured from the digests. The type III collagen content was significantly increased at the rupture site when compared to control sites (5- and 12-fold increased) or cadavers (5-fold increased). No changes in the amounts of newly synthesized type I and III procollagens were observed. The ICTP content decreased and the SP 4/ICTP ratio increased along with ageing, suggesting a structural change in the type of cross-link in the carboxyterminal telopeptide of type I collagen. Type III collagen has accumulated at the rupture site probably due to microtraumas and the subsequent healing process. The increased content of type III collagen can cause thinner collagen fibers, decrease the tensile strength and may finally result in total rupture of the tendon. The age-related change in the nature of the cross-link in the carboxyterminal telopeptide may contribute to this weakening.