Franconi's syndrome is a rare complication of the monoclonal gammopathies characterized by diffuse failure in reabsorption at the level of the proximal renal tubule resulting in glycosuria, generalized aminoaciduria, and hypophosphatemia. Current evidence suggests monoclonal light chains that are incompletely digested in renal tubule lysosomes cause renal injury. The light-chain fragments serve as a nidus for crystal formation, and the crystals interfere with a broad range of apical membrane transporters. Franconi's syndrome is predominantly associated with monoclonal kappa in the urine, but rare instances of FS caused by lambda light chains have been described. Overt hematologic malignancies such as multiple myeloma, Waldenström's macroglobulinemia, or other lymphoproliferative disorders occur in one third of patients. Clinical manifestations include slowly progressive renal failure and bone pain caused by osteomalacia. The osteomalacia is caused by chronic hypophosphatemia and may be exacerbated by secondary hyperparathyroidism and renal tubular acidosis. Treatment consists of supplementation with phosphorus, calcium, and vitamin D. The osteomalacia is often completely reversible with mineral supplementation. Patients with an associated symptomatic malignancy or rapidly progressive renal failure may also benefit from chemotherapy. The prognosis is good in the absence of overt malignant disease.