The contribution of the author’s groups on the control of energy metabolism of cancer cells has been concisely reviewed. It is proposed that Otto Warburg’s “respiratory defect” of tumoral cells resides in an alteration of the recurrent filamentation cycle of mitochondria. These give those organelles an affinity lower for ADP than the affinity of the tumor cell isoenzyme of pyruvate kinase. These two findings may explain the essentials of the intimate mechanism of the aerobic glycolysis of cancer cells.