Phosphorus plays an essential role in cellular metabolism, especially in the oxidative phosphorylation process and in the synthesis of 2-3 DPG and membrane phospholipids. Moreover phosphorus is necessary as a buffer, mainly when the organism's principal buffer, the H2CO3/HCO3- system, is working at maximal rate. The authors describe a case of severe hypophosphatemia in a ICU patient with a mixed disorder of the acid-base balance. C.P., a woman, aged 71, obese (IBW 145%), at admission in ICU showed increasing dyspnea, hypoxemia and acidosis. Besides alkaline drugs a Venturi mask with FiO2 = 0.3 alternated to CPAP cycles (7 cm H2O) with facial mask applied. Bading on CVP, MAP and ABG results, a pharmacologic therapy with enoximone, furosemide, bronchodilators, mucolytics, antacids, antibiotics and inotropics was performed. TPN with only essential amino acids was performed, in order to activate lipolysis and ketogenesis; but the ABG showed over again mixed disorder of acid-base balance (metabolic and chronic respiratory acidosis), only partially leading to ketogenesis. The reduction of the hematic HCO3-, without changes of PaCO2, was justified by the blood lactic acid of 6.2 mmol/L. And what about blood lactic acid increase? During patient hospitalization, the hematic phosphorus had decreased to, 0.8 mg/dl. Diuretic therapy together with acidosis tamponage, and reduced phosphorus feed had been responsible of severe hypo-phosphatemia. Therapy adjustments brought the phosphatemia to normal values and to a substantial improvement of clinical conditions.