Ten uraemic patients on regular haemodialysis were treated with 1alpha-hydroxycholecalciferol (1alpha-H.C.C.) for 5 to 14 months. Five patients who had histological osteitis fibrosa with or without osteomalacia responded well, with resolution of musculoskeletal pain, return of raised serum-alkaline-phosphatase concentrations to normal, resolution of radiological subperiosteal erosions, and improvement in histological signs of osteitis fibrosa and osteomalacia. In these patients 1alpha-H.C.C. proved a safe and effective drug. Five other patients did not improve. Characteristically these patients started with moderately severe histological osteomalacia and minimal, if any, osteitis fibrosa. Proximal myopathy was a prominent symptom and serum-alkaline-phosphatase was normal in four of them. Treatment with 1alpha-H.C.C. resulted in early troublesome hypercalcaemia, and repeat bone histology 5--11 months later showed no improvement. It is suggested that in these patients lack of 1,25-dihydroxycholecalciferol may not have been wholly responsible for the observed osteomalacia, hence 1alpha-H.C.C. alone was ineffective. Phosphate depeltion may have been an important contributing factor.