The concentration of the aminoterminal propeptide of type III procollagen (PIIINP) in serum as an indicator of tissue repair was studied in 71 surgical patients undergoing minor, moderate abdominal, major abdominal, or hip surgery. An increase in serum PIIINP concentrations took place within the first week, and its magnitude was related to the extent of the soft-tissue operations. After the hip replacement, the maximum PIIINP concentration was reached somewhat later. Very high levels of serum PIIINP were seen in three patients with serious wound infections. The serum PIIINP antigenicity consisted both before and after surgery of equal proportions of two forms, one corresponding to the propeptide as set free during synthesis of new collagen and the other being larger and probably derived from turnover of type III collagen fibers. The proportion of the latter form was accentuated in infection. In contrast, in the wound the form derived from synthesis of type III collagen predominated. These results suggest that the postoperative increase in serum PIIINP levels is partly the result of tissue repair and partly the result of whole-body turnover of type III collagen.