Patients with severe symptomatic renal osteodystrophy were treated with either 1,25(OH)2D3 or 1 alpha(OH)D3. In 39 instances, there was either reversal of symptoms and/or a marked fall in plasma alkaline phosphatase. Bone biopsies showed improvement of either osteomalacia or osteitis fibrosa, and serum iPTH often fell. In thirteen patients, no improvement occurred. In seven patients, bone biopsy disclosed osteomalacia, and serum iPTH was normal or only slightly elevated. Thus, there was a defect in mineralisation. apparently unrelated to the lack of 1,25(OH)2D3 and in the absence of evidence of phosphate depletion. The other 'treatment failure' group showed osteitis fibrosa on biopsy and iPTH levels were markedly elevated. They are presumed to have marked secondary hyperparathyroidism. These 'treatment failure' groups had higher pre-treatment levels of serum Ca and Mg than in those showing a favourable response; also, hypercalcaemia developed rapidly during 1,25(OH)2D3 treatment. Thus, 1,25(OH)2D3 is efficacious in treating symptomatic osteodystrophy in many uraemic patients, and in other patients, it may help identify bone disease of other, as yet unknown, pathogenesis.