We report a case of refeeding-associated hypophosphatemia in a 24-year-old malnourished male patient with anorexia nervosa but no history of alcoholism. He was given tube feeding with a low-calory preparation supplemented with phosphate. During refeeding, a severe hypophosphatemia developed after 5 days, i.e., serum phosphate 0.00-0.01 mmol/l for 2 days, accompanied by a reduction in red-cell ATP and 2,3-diphosphoglycerate, mild hemolytic anemia and transient changes in cardiac repolarization; there was, however, a striking lack of clinical symptomatology. Parenteral replacement with phosphate initially was complicated by an unexpected high urinary phosphate excretion due to an extremely low TmP/GFR (0.02 mmol/l) for over 2 days. Only after an increase of the TmP/GFR to supranormal values, i.e. up to 2.5 mmol/l, unrelated to changes in serum PTH or vitamin D3, the serum phosphate concentration became normal. The case report shows that severe hypophosphatemia can occur in nonalcoholic patients after oral feeding, and may induce reversible changes in renal phosphate handling that complicate replacement therapy.