Physical activity is associated with better health; however, the optimal intensity of activity remains unclear. A total of 13, 485 men (mean age, 57.5 years) from the Harvard Alumni Health Study reported their walking, stair climbing, and sports/recreation in 1977. Between 1977 and 1992, 2, 539 died. After adjusting for the different activity components, distance walked and storeys climbed independently predicted longevity (p, trend = 0.004 and <0.001, respectively). Light activities (<4 multiples of resting metabolic rate (METs)) were not associated with reduced mortality rates, moderate activities (4–<6 METs) appeared somewhat beneficial, and vigorous activities (≧6 METs) clearly predicted lower mortality rates (p, trend = 0.72, 0.07, and <0.001, respectively). These data provide some support for current recommendations that emphasize moderate intensity activity; they also clearly indicate a benefit of vigorous activity. Am J Epidemiol 2000; 151: 293–9.