Vitamin D deficiency is relatively common among older persons and those with spinal cord injuries. We report a case of proximal renal tubular acidosis in a 44-year-old quadriplegic nursing home resident who had limited sunlight exposure. Laboratory studies showed a low level of serum bicarbonate, a normal serum anion gap, and a positive urine anion gap. There was no history of diarrhea. Serum calcium and phosphorus levels were 6.9 mg/dL and 3.7 mg/dL, respectively. Proximal renal tubular acidosis due to osteomalacia was suspected, which was confirmed by a low serum 25-hydroxycholecalciferol level (<5 ng/mL) with an elevated serum parathyroid hormone level. Treatment with vitamin D and a calcium supplement led to normalization of laboratory values.