A prospective, randomized open-label trial of sevelamer hydrochloride with or without calcium carbonate (CC) involved 86 hemodialysis patients in Japan. The dosage of CC was fixed at 3.0 g/day for the 12-week study. After the first 4 weeks all subjects were changed from CC to sevelamer 3.0 g/day for another 4 weeks, then allocated randomly to three groups for the final 4 weeks: group A, sevelamer 6.0 g/day; group B, sevelamer 3.0 g/day and CC 3.0 g/day; group C, CC 3.0 g/day. The target serum phosphorous concentration (P)=5.5 mg/dL and the corrected calcium concentration (Ca) was 9.0-10.0 mg/dL. Of the 86 patients, 62 finished the study without a change of dosage and their data were analyzed (group A, N=16; group B, N=26; group C, N=20). At week 8 compared with week 4, the concentration of P increased from 5.7+/-1.4 to 6.4+/-1.7 mg/dL in group A, and decreased significantly in groups B and C, and in group B compared with groups A and C; groups A and C had similar concentrations at week 8. The Ca concentration decreased significantly from 9.7+/-1.0 to 9.1+/-0.7 mg/dL after the change to sevelamer. At week 8 Ca was not significantly changed in group A, whereas a significant increase occurred in groups B and C. Side-effects with sevelamer administration occurred in 34 of the 86 patients and 24 dropped out of the study, with a high frequency in group A (13/29; 44.8%). In conclusion, there was an additive effect of sevelamer for the treatment of hyperphosphatemia with CC. The combination therapy was better tolerated and showed higher patient compliance than CC or sevelamer monotherapy.