Experimental evidence suggests that renal 1 alpha-hydroxylase activity is impaired in Fanconi syndrome. We have evaluated plasma vitamin D metabolites in five patients with Fanconi syndrome, three of whom had metabolic bone disease; plasma 1,25(OH)2D3 was low in the three patients with bone disease, and normal in the two patients without a bone mineralisation defect. The data supports the hypothesis that renal 1 alpha-hydroxylase activity may be impaired in human Fanconi syndrome, and that altered vitamin D metabolism may contribute to the pathogenesis of metabolic bone disease in Fanconi syndrome.