We evaluated the subsequent loss of bone from the proximal part of the ipsilateral and contralateral femora and from the lumbar spine of seven men and nine women who had a fracture of the tibia. The average age was sixty years. All of the fractures were unstable, and the involved leg bore no weight for an average of eight weeks. The bone mineral density was measured with dual-energy x-ray absorptiometry of the lumbar spine and of the femoral neck and the trochanteric region of both hips immediately after the fracture, after the period of immobilization, and at approximately three, six, and twelve months after the fracture. During the period of immobilization, the bone mineral density of the trochanteric region decreased an average of 9 +/- 7 per cent on the side of the fracture, compared with the value immediately after the fracture, but there was no change on the contralateral side (p < 0.01). At twelve months, the average decrease in the trochanteric area was 15 +/- 10 per cent on the side of the fracture, compared with the value immediately after the fracture, but again there had been no change on the uninjured side (p < 0.01). The bone mineral density of the femoral neck on the side of the fracture had decreased 6 +/- 6 per cent at twelve months, compared with a decrease of 2 +/- 4 per cent on the uninjured side (p < 0.05). The bone mineral density of the lumbar spine decreased only during the period of unloading of the fractured leg (1 +/- 2 per cent, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)