To test the hypothesis that weight loss in older men is associated with increased rates of hip bone loss regardless of adiposity and intention to lose weight, we measured body weight, body composition, hip bone mineral density (BMD), and intention to lose weight in a cohort of 1342 older men enrolled in the Osteoporotic Fractures in Men (MrOS) study and followed them prospectively for an average of 1.8 yr for changes in weight and BMD. The adjusted average rate of change in total hip BMD was 0.1%/yr in men with weight gain, -0.3%/yr in men with stable weight, and -1.4%/yr in men with weight loss (test for trend, P < 0.001). Higher rates of hip bone loss were observed in men with weight loss regardless of category of body mass index, body composition, or intention to lose weight. Even among obese (body mass indes, ≥30 kg/m2) men trying to lose weight, those with documented voluntary weight reduction experienced an increase in hip bone loss (average rate of change in total hip BMD, 0.5%/yr in those with weight gain, -0.1%/yr in those with stable weight, and -1.7%/yr in those with weight loss; test for trend, P < 0.001). Older men who experience weight loss have increased rates of hip bone loss, even among overweight and obese men undergoing voluntary weight reduction.