Adult-onset osteomalacia with multiple renal tubular defects and generalized aminoaciduria is uncommon, and where familial it is characteristically an autosomal recessive disorder. This paper describes a kindred in which the syndrome has appeared in four successive generations, apparently inherited in a dominant manner, and possibly associated with diabetes mellitus. The proposita had hypophosphataemia, renal glycosuria, proteinuria and generalized aminoaciduria, and at the age of 22 developed symptoms of osteomalacia which responded to treatment with oral phosphate. Her father had been similarly affected: renal glycosuria was first noted when he was 24, and 12 years later he developed diabetes mellitus from which he died. One sister, aged 31, has renal glycosuria, aminoaciduria and hypophosphataemia without bone disease. In the three preceding generations at least seven other individuals had crippling bone disease and profound muscle weakness of early adult onset; in four, preterminal polydipsia was recorded, and others had renal glycosuria or diabetes mellitus. Three of the five children in the latest generation have slight proteinuria but not other detectable abnormality. The possible association between these renal tubular defects and diabetes mellitus is discussed.