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A 71 year old hypertensive and non insulin-dependent diabetic patients with moderate renal insufficiency taking 500 mg/d of metformin and 5 mg/d of enalapril, developed metabolic acidosis characterized by fairly elevated anion gap, hyperchloremia, severe hyperkalemia, normal plasma level of beta-hydroxybutyric acid, absence of ketonuria and high plasma level of lactic acid. This biochemical feature allowed us to ascribe the pathogenesis of metabolic acidosis both to the increased plasma level of lactic acid and to the type IV renal tubular acidosis syndrome, the precipitating factor being an infection of urinary tract (as we assumed on the basis of the urine culture). The patient was dehydrated and lethargic; the ECG revealed the presence of nonparoxysmal junctional tachycardia. The clinical evolution was favorable thanks to the treatment with the infusion of isotonic saline solutions, mild alkalinizing solutions, low-dose regular insulin and antibiotics. It is likely that metformin and enalapril, regularly assumed by the patient, could have played a iatrogenic role even if they were taken in low dosages. This event points out the importance of complying with the indications and especially the contraindications of these drugs, to avoid life threatening complications as that one occurred in this case.