Hypophosphatemic osteomalacia is a rare but important complication of multiple myeloma. In these cases, the pathophysiology of the phosphate renal wasting notably differs from oncogenic osteomalacia and is due to light-chain nephropathy, resulting in proximal tubular dysfunction which is not restricted to phosphate handling. These patients seems to have a distinct type of plasma cell disorder characterized by a slow progression of the tumor and by an early phase dominated by the metabolic complications of the renal proximal tubular dysfunction. For this reason hypophosphatemic osteomalacia is the presenting feature that leads to the diagnosis of multiple myeloma in most of these patients. Recognition of this complication is important, since supportive treatment with phosphate supplements and calcitriol may substantially alleviate pain and weakness associated with hypophosphatemia.