Two patients with Kearns-Sayre Syndrome and hypoparathyroidism were treated with alfacalcidol (1a-OH D3) and total serum calcium concentration remained within normal range for a long period. After two months of combined therapy with Coenzyme Q10 (CoQ10), hypercalcemia was noticed and as a result, 1a-OHD3 was gradually discontinued. Normal total serum calcium concentration was obtained with CoQ10 monotherapy while the replacement of CoQ10 with placebo led to hypocalcemia. The mechanism of action of CoQ10 is difficult to explain. Since the parathormone level remained unchanged during CoQ10 or placebo therapy, we speculate that the capacity of producing an active form of vitamin D in mitochondria of proximal tubules was restored by CoQ10 therapy.