We measured peritoneal losses of the active vitamin D metabolites 1,25(OH)2D3 and 24,25(OH)2D3 in patients receiving continuous ambulatory peritoneal dialysis (CAPD). The serum concentration of 24,25(OH)2D3 was considerably lower than in hemodialysis patients. The serum concentration of 1,25(OH)2D3 was undetectable and rose to levels similar to those in hemodialysis patients only after loading with much higher oral doses of 1-α-vitamin D3 than those received by hemodialysis patients. Losses of both metabolites in peritoneal fluid were considerable, averaging approximately 6–8% of the plasma pool per day. These losses lead to low serum levels of these active vitamin D metabolites in CAPD patients, which may be an important factor in exacerbating renal osteodystrophy. Our results indicate the need for increased replacement doses of vitamin D metabolites in CAPD patien