The purpose of this study is to compare the proportion of chronic kidney disease (CKD) subjects on dialysis receiving Sensipar® and low dose vitamin D or escalating doses of vitamin D alone who are able to achieve the National Kidney Foundation-Kidney Disease Outcomes Quality Initiative (NKF-K/DOQI) secondary hyperparathyroidism (HPT) treatment targets for both biointact parathyroid hormone (biPTH) and calcium-phosphorus (Ca x P) product.