BACKGROUND Hyperphosphatemia is a common problem in patients with kidney failure. It is usually mild and rarely severe enough to cause metabolic acidosis on its own. Besides kidney failure, use of phosphate containing enemas, rhabdomyolysis, and tumor lysis syndrome are common causes of severe hyperphosphatemia. CASE REPORT A 74-year-old man with a history of diabetes mellitus type II, arterial hypertension, and end stage renal disease, who was on hemodialysis and who had undergone hemicolectomy for ischemic bowel disease, and had not eaten for several days, developed severe metabolic acidosis, with an anion gap (AG) of 31 meq/L, -uncorrected for serum albumin. At that time he had a high level of beta-hydroxybutyrate and severe hyperphosphatemia (16.5 mg/dL). Metabolic acidosis and hyperphosphatemia were corrected with hemodialysis, confirming the role of hyperphosphatemia in the development of high AG metabolic acidosis. CONCLUSIONS Although our patient had many reasons to develop high AG metabolic acidosis, hyperphosphatemia played a significant role in his acidosis. Severe hyperphosphatemia is rarely mentioned as a cause of high AG acidosis. It should be added to the long list of causes of this metabolic disorder. Physiological basis of acid base changes are discussed.