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The case records of 327 patients who underwent bone biopsy in late or terminal renal failure, before any form of dialysis or transplantation, were examined for clues to the aetiology of renal osteomalacia and its manifestations. Fifty four per cent of the biopsies showed pure osteitis fibrosa, 34 per cent osteomalacia with osteitis fibrosa and 12 per cent showed neither abnormality. Osteomalacia was strongly associated with chronic pyelonephritis and obstructive uropathy as primary renal disease. In two matched groups of 100 each, and within the major primary diseases, it was associated with acidosis, hypocalcaemia and normophosphataemia (as opposed to hyperphosphataemia). There was no association with known length or uraemia and only a weak and inconsistent relationship with severity of uraemia. In the few patients studied, there was no relationship between osteomalacia and serum 25-hydroxycholecalciferol level. In contrast to the state of patients treated by haemodialysis, osteomalacia in this undialysed group was manifested by a higher level of serum alkaline phosphatase than pure osteitis fibrosa, serum iPTH did not differ between the groups, there was no predominance of symptoms in one group, other than proximal myopathy which had a weak association with osteomalacia, and Looser zones were more common than complete fractures. Our study shows that osteomalacia has different manifestations, and probably different causes, before and after the start of haemodialysis. These two stages of renal failure should be clearly distinguished in reports of renal bone disease.