In the Plymouth area, 95 patients with end-stage renal failure have undergone haemodialysis for 6 months or longer. Of the 47 patients beginning dialysis between 1967 and 1973, when water deionisers were not used routinely, a bone disease with multiple fractures, ‘fracturing osteodystrophy’, occurred in 18 patients and dialysis encephalopathy in 10. Of the 48 patients first dialysing between 1974 and 1979, when water deionisers were used commonly, fracturing osteodystrophy occurred in only one and dialysis encephalopathy also in only one. Duration of dialysis without a water deioniser appeared to be the mostimportant factor in the development of these two conditions. The use of water deionisers usually led to healing of fractures in patients with fracturing osteodystrophy and also led to improvement in 4 of the 11 patients with dialysis encephalopathy. Neither condition has occurred in any patient using a water deioniser from the first dialysis. Water deionisers, therefore, appeared to be effective in both the treatment and prevention of fracturing osteodystrophy and dialysis encephalopathy.