The effects of one year of exercise training on cardiorespiratory fitness, levels of daily leisure activity, and blood lipids (cholesterol and high density lipoproteins) were studied in a prospective, randomized clinical trial. Two hundred and twenty-four men aged 55 to 65 years volunteered for the study and were randomly allocated to a control (n = 111) or an activity (n = 113) group with stratification on blue or white collar job classification. After the attrition due to loss to follow-up, 100 men remained in each of the control and activity groups. The exercising men met an average of 2.5 times per week over the year and their VO2 max or peak VO2 (ml X kg-1 X min-1) increased significantly (p = .001, 11%) compared with controls. There were no significant changes in maximal heart rate (155 bpm) and respiratory exchange ratio (1.1), although ventilation (80 to 90 l X min-1) increased significantly in the trained group. In addition, the VO2 at a heart rate of 125 bpm increased significantly (p less than .001) in the trained group (14.7%) over that observed in the control (1.9%). There were no significant differences between the groups with respect to the remaining end-points.