The aim of this study was to evaluate the effect of decreased arm function after stroke on bone mineral density (BMD) in the proximal humerus. Twenty-five patients were evaluated 1 week after acute stroke and reevaluated 2 months, 7 months, and 1 year after the stroke. BMD of the proximal humerus was measured and the functional status of the paretic arm was assessed. Within 1 year the BMD decreased significantly in the proximal humerus at the paretic side. No significant BMD change was found at the nonparetic side. Patients with an initial completely paralyzed arm lost 27% (P <0.001), those with severe to moderate paresis lost 11% (P <0.001), and patients with minor or no paresis had no significant bone loss in the proximal humerus at the paretic side after 1 year. A statistically significant trend with initial arm function was found both 7 months and 1 year after the stroke at the paretic side (P <0.01). Patients who were paralyzed or had severe to moderate paresis both at inclusion and after 1 year had a larger 1-year bone loss in the proximal humerus than the patients who recovered and these who had only minor impairment throughout the study, 25%, 8%, and 5%, respectively (P-value for linear trend <0.001). We conclude that during the first year after stroke bone mineral is lost in the proximal humerus of the paretic arm, but that the loss depends on the initial degree of paresis. The loss may be prevented if arm function is regained.