This meta-analysis of controlled clinical trials quantifies the effect of aerobic exercise on VO2max (aerobic fitness) among sedentary older adults and examines the associations of such magnitude with variables of intervention. Forty-one trials including 2102 older subjects (within-group mean age of 60 years and older) were identified by searches of databases, hand searching, and cross-referencing. Outcomes were homogeneous. The pooled standardized effect size by a fixed-effect model showed a higher moderate effect (mean +/- SEM) of 0.64+/-0.05; 95% confidence interval, 0.56-0.73; p<0.001, representing a net increase in VO2max (mean +/- SEM) of 3.78+/-0.28 mL.kg-1.min-1; 95% confidence interval, 3.24-4.33; or a 16.3% improvement, compared with control groups. Greater improvement in VO2max was associated with training length more than 20 weeks and training intensity of approximately 60% but less than 70% of VO2max. Endurance training improves aerobic fitness in older adults, thus providing protective benefits for cardiovascular aging and quality of later life.