Stroke patients have increased risk of hip fractures. Nearly all fractures occur on the hemiplegic side, and reduced bone mineral density (BMD) may be an important predisposing factor. The aim of this study was to investigate the degree of demineralization within the first year after stroke, and to elucidate a possible difference in patients with high versus low ambulatory levels. Forty acute stroke patients were followed (17 initially wheelchair-bound and 23 initially ambulatory). BMD was measured in the proximal femur bilaterally at a mean 6 days, 7 months and 1 year after stroke onset using dual-energy X-ray absorptiometry. Ambulatory status was independently associated with changes in BMD (p < or = 0.005) 1 year after stroke. The 17 initially wheelchair-bound patients had a significant 10% reduction in BMD at the paretic side and 5% reduction at the non-paretic side (p < 0.001), while the 23 patients initially able to walk had a significant loss (3%) only at the paretic side (p = 0.01). The analysis also indicated that the major reduction in BMD took place within the first 7 months. Two months after stroke 12 of the wheelchair-bound patients had relearned to walk. At the paretic side the 1 year changes in BMD in the patients who stayed wheelchair-bound, the patients who re-learned to walk within the first 2 months and the patients who were able to walk throughout the study were 13%, 8% and 3%, respectively, and a statistically significant trend with ambulatory level was found (p = 0.007). This study provides clear evidence that lack of mobility and weight-bearing early after stroke is an important factor for the greater bone loss in the paretic leg, but that relearning to walk within the first 2 months after stroke, even with the support of another person, may reduce the bone loss after immobilization.