The authors report the case of a patient with chronic obstructive pulmonary disease (COPD) whose renal function depends on the ventilatory pattern. The data appeared in opposition with the standard physiopathological notions with an improvement of renal function during mechanical ventilatory assistance and a degradation while spontaneous breathing. The authors suggest an original hypothesis based upon the increase of oxygen cost of breathing (OCB) with a redistribution of the intravascular blood flow to the diaphragm.