Lower dialysate calcium concentrations were recently proposed to overcome the risk of hypercalcemia in continuous ambulatory peritoneal dialysis (CAPD) patients on calcium-containing phosphate binders and/or vitamin D metabolites using the standard dialysate calcium concentration (SCa) of 1.75 mM. To assess transperitoneal calcium mass transfer (CaMT) in CAPD patients using a dialysate with a low calcium concentration (LCa, 1.00 mM), 18 stable patients were randomly allocated to receive either LCa or SCa. CaMT was assessed over 4 hours using 2L dialysate bags with three different dialysate glucose concentrations (1.5%, 2.3%, 4.25%). Total serum calcium (tCa), ionized calcium (iCa), and the exact dialysate volume were measured before and after the 4-hour dwell. A sample of the drained dialysate was obtained to measure the dialysate calcium concentration. The tCa and iCa levels were not significantly different in both groups prior to and did not change throughout the test. CaMT (median/range) was -0.64 mmol/exchange (-0.35(-)-1.29 mmol/exchange) using LCa with 1.5% glucose compared to 0.23 mmol (-0.18-0.87 mmol) with SCa (p < 0.0001). CaMT was negatively correlated to iCa and ultrafiltration volume [4.25%: LCa-1.22 (-0.84(-)-1.9); SCa -0.43 (-1.35-0.13); p < 0.001]. In summary, LCa results in a loss of calcium into the dialysate even at low ultrafiltration volumes and serum iCa levels. This might facilitate the prevention and therapy of renal osteodystrophy with calcium-containing phosphate binders and calcitriol. However, patients using LCa must be carefully monitored for calcium homeostasis and bone turnover.