Although deficits in skeletal muscle strength, gait, balance, and oxygen uptake are potentially reversible causes of frailty, the efficacy of exercise in reversing frailty in community-dwelling older adults has not been proven. The aim of this study was to determine the effects of intensive exercise training (ET) on measures of physical frailty in older community-dwelling men and women. Randomized controlled trial. Medical school research center. One hundred fifteen sedentary men and women (mean age +/- standard deviation = 83 +/- 4) with mild to moderate physical frailty, as defined by two of the following three criteria: Modified Physical Performance Test (modified PPT) score between 18 and 32, peak oxygen uptake (VO2 peak) between 10 and 18 mL/kg/min, and self-report of difficulty or assistance with one basic activity of daily living (ADL), or two instrumental ADLs. Participants were randomly assigned to a control group that performed a 9-month low-intensity home exercise program (control) or an exercise-training program (ET). The control intervention primarily consisted of flexibility exercises. ET began with 3 months of flexibility, light-resistance, and balance training. During the next 3 months, resistance training was added, and, during the next 3 months, endurance training was added. Modified PPT score, VO2 peak, performance of ADLs as measured by the Older Americans Resources and Services instrument, and the Functional Status Questionnaire (FSQ). ET resulted in significantly greater improvements than home exercise in three of the four primary outcome measures. Adjusted 95% confidence bounds on the magnitude of improvement in the ET group compared with the control group were 1.0 to 5.2 points for the modified PPT score, 0.9 to 3.6 mL/kg/min for VO2 peak, and 1.6 to 4.9 points for the FSQ score. Our results show that intensive ET can improve measures of physical function and preclinical disability in older adults who have impairments in physical performance and oxygen uptake and are not taking hormone replacement therapy better than a low-intensity home exercise program.