Serum osteocalcin has been found to correlate with bone formation. However, present literature gives only limited data on osteocalcin and bone histomorphometry in patients undergoing peritoneal dialysis. This study assessed serum osteocalcin, dialysate osteocalcin, peritoneal clearance of osteocalcin (Cl(p)-osteocalcin) and mass transfer of osteocalcin (MT(p)-osteocalcin), and evaluated relationships between these values and bone histomorphometry. Eighteen patients were treated by continuous ambulatory peritoneal dialysis (CAPD). Bone biopsies, serum and dialysate osteocalcin, serum levels of parathyroid hormone, alkaline phosphatase, aluminum, phosphate, Ca2+ and vitamin D3 metabolites were measured at the start and in 10 of the patients a year later. Serum osteocalcin was found to be elevated. Osteocalcin was detected in the dialysate resulting in significant values of Cl(p)-osteocalcin and MT(p)-osteocalcin. Serum and dialysate levels of osteocalcin correlated significantly (r = 0.66, P < 0.001) and like MT(p)-osteocalcin with serum levels of alkaline phosphatase and PTH. Histomorphometry showed that osteitis fibrosa was the predominant bone disease detected. Serum concentration of osteocalcin correlated with osteoid thickness, eroded and osteoclast surfaces, aluminum staining, and some of the bone dynamic parameters. Dialysate osteocalcin, MT(p)-osteocalcin, PTH and alkaline phosphatase correlated with practically the same histomorphometric parameters as serum osteocalcin. No correlations were seen between Cl(p)-osteocalcin and any histomorphometric parameters. Serum osteocalcin was elevated above the normal range, and significant positive correlations between serum osteocalcin and bone formation parameters were found. Serum osteocalcin correlated with almost the same histomorphometric parameters as PTH. Thus, serum levels of PTH and osteocalcin gave additional information to one another as non-invasive parameters in this group of patients. Hence, serum osteocalcin is a valuable non-invasive index of metabolic bone disease in patients treated by CAPD. The transperitoneal removal of osteocalcin does not appear to be clinically significant.