Introduction: There is little evidence as to whether exercise can increase BMD in older men with no investigation of high impact exercise. Lifestyle changes and individual variability may confound exercise trials but can be minimised using a within-subject unilateral design (exercise leg [EL] vs. control leg [CL]) that has high statistical power. Purpose: This study investigated the influence of a 12. month high impact unilateral exercise intervention on femoral neck BMD in older men. Methods: Fifty, healthy, community-dwelling older men commenced a 12. month high impact unilateral exercise intervention which increased to 50 multidirectional hops, 7. days a week on one randomly allocated leg. BMD of both femurs was measured using dual energy X-ray absorptiometry (DXA) before and after 12. months of exercise, by an observer blind to the leg allocation. Repeated measures ANOVA with post hoc tests was used to detect significant effects of time, leg and interaction. Results: Thirty-five men (mean ± SD, age 69.9 ± 4.0. years) exercised for 12. months and intervention adherence was 90.5 ± 9.1% (304 ± 31 sessions completed out of 336 prescribed sessions). Fourteen men did not complete the 12. month exercise intervention due to: health problems or injuries unrelated to the intervention (n = 9), time commitments (n = 2), or discomfort during exercise (n = 3), whilst BMD data were missing for one man. Femoral neck BMD, BMC and cross-sectional area all increased in the EL (+ 0.7, + 0.9 and + 1.2 % respectively) compared to the CL (-0.9,. 0.4 and. 1.2%); interaction effect P<0.05. Although the interaction term was not significant (P>0.05), there were significant main effects of time for section modulus (P=0.044) and minimum neck width (P = 0.006). Section modulus increased significantly in the EL (P = 0.016) but not in the CL (P = 0.465); mean change + 2.3% and + 0.7% respectively, whereas minimum neck width increased significantly in the CL (P = 0.004) but not in the EL (P = 0.166); mean changes being + 0.7% and + 0.3% respectively. Conclusion: A 12. month high impact unilateral exercise intervention was feasible and effective for improving femoral neck BMD, BMC and geometry in older men. Carefully targeted high impact exercises may be suitable for incorporation into exercise interventions aimed at preventing fractures in healthy community-dwelling older men.