We measured peritoneal losses of the active vitamin D metabolites 1,25(OH)<sub>2</sub>D<sub>3</sub> and 24,25(OH)<sub>2</sub>D<sub>3</sub> in patients receiving continuous ambulatory peritoneal dialysis (CAPD). The serum concentration of 24,25(OH)<sub>2</sub>D<sub>3</sub> was considerably lower than in hemodialysis patients. The serum concentration of 1,25(OH)<sub>2</sub>D<sub>3</sub> was undetectable and rose to levels similar to those in hemodialysis patients only after loading with much higher oral doses of 1-α-vitamin D<sub>3</sub> 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 patients.