The carboxyterminal propeptide of type I procollagen (PICP) and the aminoterminal propeptide of type III procollagen (PIIINP) are split products related to formation of granulation tissue. Local concentrations reflect the state of fibrosis activity. PICP and PIINP were measured in serum and dialysate from 13 CAPD patients. One patient had 5-fold higher PICP and 8-fold higher PIIINP in the dialysate than the 12 other patients receiving CAPD. One month after the samples were collected, the patient was taken off CAPD due to sclerosins peritonitis. Our observation indicates that determinations of PICP and PIIINP in the dialysate could be useful markers of peritoneal fibrosis.