Secondary hyperparathyroidism is a complex pathophysiologic event which we examined in hundred patients Sixteen of them belonged to the control with normal renal function, and the rest of them were categorised into four subgroups according to the actual level of chronic renal failure, determined by GFR a.i. by endogenous creatinine clearance. We determined the serum level of parathormone (C-PTH) and plasma concentration of phosphate and calcium in all of patients. In 43 of them we measured serum level of 1.25 dihydroxycholecalcipherol and ionised calcium. Our results suggested an important parallelism between parathormone hypersecretion (GFR +/- 58.3 ml/min), 1.25 dihydroxycholecalcipherol hypovitaminosis (GFR +/- 50.7 ml/min) and hypocalcemia (GFR +/- 47.7 ml/min) which connect them in very strong relationship. Even if secondary hyperparathyroidism in chronic renal failure is caused by numerous factors, it appears that the hypovitaminosis of 1.25 dihydroxycholecalcipherol with consequent hypocalcemia and phosphate retention acts as a main movement of this complex pathophysiology mechanism.