Background Physical activity and adiposity are important predictors of mortality, even in older individuals. However, it is unclear how much physical activity is needed to prevent weight gain in older persons. Purpose To examine the associations of different amounts of physical activity with weight gain prevention in older men. Methods 5,973 healthy men (mean age, 65.0 y) from the Harvard Alumni Health Study were followed from 1988 to 1998. At baseline (1988), in 1993, and 1998, men reported their recreational physical activity and body weight. Physical activity was categorized as: <7.5 MET-hr/week (7.5 MET-hr/week corresponds to the minimum required by the 2008 US federal guidelines), 7.5 to <21 MET-hr/week (21 MET-hr/week corresponds to the 2002 Institute of Medicine [IOM] guideline), and ≥21 MET-hr/week. Meaningful weight gain was defined as an increase of ≥3% of body weight. Results Overall, weight tended to be stable over any 5-year period; mean change, −0.08 (SD=4.44) kg. However, ~21% of men experienced meaningful weight gain over any 5-year period. In multivariate analyses, compared to men expending ≥21 MET-hr/week, those expending 7.5 to <21 MET-hr/week had an odds ratio (OR) of 1.35 (95% confidence interval: 1.03, 1.77) for meaningful weight gain, and men expending <7.5 MET-hr/week, an OR of 1.16 (1.01, 1.33) (p, trend = 0.09). Conclusions Among older men, those with lesser levels of physical activity were more likely to gain weight than men satisfying the 2002 IOM guidelines of ≥21 MET-hr/week (~60 minutes per day of moderate-intensity physical activity).