Bone is a complex, organized tissue that consists of both, mineral and organic substances. It is also a highly dynamic tissue that is manifested by continuous bone remodeling. Osteoblasts are bone cells that create new bone, while osteoclastic bone cells are responsible for the resorption of the already created bone. OPG/RANKL/RANK system plays a key role in bone remodeling regulation. This system is consisting of the RANKL glycoprotein molecule, its related RANK receptor, and OPG as the "decoy receptor''. Renal osteodystrophy is a name for various pathophysiological conditions affecting the bone of patients with chronic renal failure. The main pathophysiological mechanisms present in renal osteodystrophy are hyperphosphatemia, hypocalciemia, with vitamin D synthesis disorders. According to the results of this study, patients on hemodialysis had significantly higher levels of AP, OPG, sRANKL and iPTH compared to transplanted kidney recipients (Tx12) and those with no renal disease. Patients on hemodialysis and transplanted kidney recipients on vitamin D therapy had significantly higher levels of OPG, with significantly lower sRANKL levels. These results suggest the beneficial effect of vitamin D on bone mass loss prevention in patients on hemodialysis and in kidney transplant recipients. The RANKL/OPG serum level ratio could be better correlated with iPTH values than a separate measurement of these factors, but it remains unclear why the RANKL/OPG ratio is low in moderately elevated iPTH values and further research is needed in order to get the answer to this question.