Early reports of patients with metabolic bone diseases such as nutritional osteomalacia, Fanconi syndrome, indicated an association with aminoaciduria. This association has since been described in osteomalacia of G. I. or hepatic origin, secondary to anticonvulsant therapy, tumors, and chronic renal failure. Aminoaciduria also occurs in primary hyperparathyroidism. In nutritional osteomalacia, vitamin D deficiency was thought to be responsible for the renal tubular abnormality, since it responded to treatment with vitamin D. However, since the description of aminoaciduria in hyperparathyroidism, the literature has been divided concerning the etiology of aminoacidura in conditions associated with abnormal vitamin D metabolism because secondary hyperparathyroidism often occurs in these conditions. Recently, some cases of Fanconi syndrome and a case of tumor-associated osteomalacia have been described with low or absent plasma 1,25-dihydroxycholecalciferol levels, normal serum PTH, and aminoaciduria. In one of these cases, and more recently in patients with chronic renal failure, it has been demonstrated that treatment with 1,25(OH)2D3 can improve amino acid transport independently from changes in serum PTH levels. 1,25(OH)2D3 therefore normally opposes the aminoaciduric effect of PTH. This is an agreement with observations which demonstrate that 1,25(OH)2D3 also opposes the phosphaturic action of parathyroid hormone.