Serum procollagen type I carboxyterminal propeptide (PICP) has been shown to be a useful marker of bone formation in patients undergoing haemodialysis. However, PICP levels has not been evaluated in depth in patients maintained on continuous ambulatory peritoneal dialysis (CAPD). Therefore serum and dialysate levels of PICP, its peritoneal clearance (Clp), mass transfer (MTp), and its possible relationship with osteocalcin, parathyroid hormone (PTH), and bone histomorphometry were studied in a group of CAPD patients. Serum PICP was just above the normal range with significant amounts detected in the dialysate but no correlations were found between levels of serum PICP, dialysate PICP, and Clp-PICP. One patient with systemic lupus and osteitis fibrosa had extraordinarily high serum and dialysate levels of PICP. The patient later developed sclerosing peritonitis. No associations were seen between serum PICP and Clp-PICP and any of the 18 bone histomorphometric parameters evaluated. Dialysate level of PICP correlated negatively with mineral appositional rate (r = -0.62, P < 0.01) and mineralization lag time (r = 0.64, P < 0.01). MTp-PICP correlated positively with mineral appositional rate (r = 0.65, P < 0.01). Serum osteocalcin and serum PTH levels did not correlate to serum, dialysate, Clp or MTp measurements of PICP. These results suggest that measurements of PICP in CAPD patients do not give substantial information as an non-invasive marker of bone histology.