In briefTo assess the decline in maximal aerobic power (V O2max) with age, two groups of men were studied: Fifteen exercisers were followed from age 45 to 68, and 15 controls were measured while training at age 52 and again at age 70 after being detrained for 18 years. Resting blood pressure of 120/78 was unchanged in the exercisers but rose from 135/85 to 150/90 in the nonexercisers. V O2max measured by open circuitry declined 13% in the exercisers from 44.4 to 38.6 ml-min−1-kg−1; in the nonexercisers it declined 41% from 34.2 to 20.3 ml-min−1·kg−1. The VOmax loss was primarily due to a combined loss of maximal heart rate and stroke volume. The data suggest that regular aerobic exercise retards the usual loss in aerobic power with age and present important implications for the aging population in relation to social, economic, and health benefits.