The viability of osteocytes can be demonstrated in sawn decalcified sections of bone by their lactate dehydrogenase activity. In the cancellous bone of the femoral head, the proportion of lacunae containing viable osteocytes decreased from 88 +/- 7% (mean +/- SD) at 10-29 years to 58 +/- 12% (P < 0.001) by 70-89 years. Viability in the second lumbar vertebra was 88 +/- 3% in subjects aged 25-90 years and did not decrease with age. Mean osteocyte viability in the femoral head of 21 hip fracture patients aged 72-94 years was 58 +/- 21%, similar to controls of a similar age, though there was greater variation and, in five patients, osteocyte viability was less than 25%. In hip fracture patients, microfracture callus incidence correlated positively with osteocyte viability, with little or no fracture callus observed if the bone viability was low. Ultimate compressive strength did not correlate with osteocyte viability. In the femoral head there is gradual, age-related reduction in osteocyte viability that can be more pronounced in hip fracture. Osteocyte death may affect bone quality by impairing repair of fatigue damage.