The reduced bone mineral density (BMD) found in patients with fractures may, in part, follow rather than precede the fracture. We studied the magnitude and reversibility of bone loss in the 15 months following osteotomy in 21 men and 5 women with localized medial arthritis of the knee. BMD (mean +/- SD), measured using dual-energy X-ray absorptiometry, decreased by a maximum of 35 +/- 21% in the mid-diaphysis of the affected tibia at 9 months after surgery (p < 0.001). At 15 months, reversal of bone loss in non-fractured bones was incomplete; the remaining deficit was 20 +/- 27% relative to baseline (p < 0.001). Maximum bone loss occurred at 9 months at the total body (5 +/- 2%), spine (15 +/- 17%) and at Ward's triangle of the proximal femur of the unoperated limb (10 +/- 17%) (all p < 0.01). In summary, post-traumatic bone loss is region-specific with incomplete reversibility, at least after about 15 months. Deficits in BMD in cross-sectional studies of patients with fractures, held to be responsible for the bone fragility, may, in part, follow rather than precede the fracture.