OBJECTIVE To determine whether it is practicable to use intraperitoneal calcitriol to treat continuous ambulatory peritoneal dialysis (CAPD) patients with secondary hyperparathyroidism and whether this form of therapy is effective and safe. METHODS A prospective, nonrandomized study. METHODS Division of Nephrology, Tertiary Hospital. METHODS Eight patients from our CAPD population were recruited (5 F, 3 M), aged 24 to 63 years (mean 38.9 +/- 7.6 yr). They had been on CAPD for 8 to 84 months (mean 47.6 +/- 24.6 months). All the patients had bone biopsy-proven secondary hyperparathyroidism with 2 patients showing mixed lesions. The CAPD system was changed to the twin-bag system (Ultrabag, Baxter Healthcare, McGaw Park, IL, U.S.A.) in all 8 patients, who were taught to inject the calcitriol directly through the short transfer set and the Tenckhoff catheter into the peritoneal cavity, twice per week before bedtime. Calcium carbonate or calcium acetate was used as the main phosphate binder. Intact parathyroid hormone level (iPTH), serum ionized calcium (iCa), serum phosphate, and serum total alkaline phosphatase (alk. phos.) levels were measured at baseline and then every 4 weeks. The mean duration of follow-up was 10.5 months +/- 1.9 months. RESULTS There was a significant drop of iPTH level from the pretreatment level of 100.6 +/- 35.8 pmol/L to a level of 63.8 +/- 48.7 pmol/L at 24 weeks (p = 0.036). The lowest level of iPTH attained was 43.4 +/- 27.0 pmol/L at 48 weeks. Serum total alk. phos. also dropped from 232.4 +/- 83.3 IU/L pretreatment to 147.9 +/- 52.0 IU/L at 24 weeks (p = 0.017). The decrease in alk. phos. level paralleled the decrease in iPTH level. The mean serum iCa level did not show any significant rise throughout the study period. The maximum dose of calcitriol used was 6.6 +/- 1.5 microg/week and the average dose of calcitriol was 5.4 +/- 1.2 microg/week. One patient did not respond satisfactorily and she subsequently had a parathyroidectomy. Two episodes of peritonitis occurred during the study period, giving a peritonitis rate of one episode per 42 patient-months. There was no significant change in the urea clearance tests or the peritoneal equilibration tests before and after the study. CONCLUSIONS Intraperitoneal calcitriol is practicable, effective, and safe in the treatment of secondary hyperparathyroidism in CAPD patients.