We investigated whether 30 days of oral supplementation with a special mixture of amino acids (AAs), together with conventional therapy, could improve exercise capacity in elderly outpatients with chronic heart failure (CHF). A group of 95 outpatients (12 women and 83 men; New York Heart Association class II–III) aged 65–74 years were studied. This was a randomized, double-blind, placebo-controlled study. The patients performed a basal exercise test and were then randomly assigned to a special oral nutritional mixture of AAs 4 g twice daily (n = 43) or placebo (n = 42). After 30 days we repeated the exercise test. In both tests we measured the following: oxygen consumption (VO₂), CO₂ production (VCO₂), minute ventilation (VE), oxygen cost of ventilation (VO₂/VE), CO₂ elimination per liter of ventilation (VCO₂/VE), respiratory exchange ratio (RER; calculated as VCO₂/VO₂), oxygen pulse (VO₂/heart rate [HR]) and anaerobic metabolism during exercise (ANA-VO₂). At day 30, exercise capacity in the AA group had improved (+11 ± 8 W, p <0.01; +67.5 ± 44 seconds, p <0.02). This improvement was associated with both reduced circulatory dysfunction and increased peripheral oxygen availability. Indeed, peak VO₂ increased by 1.2 ± 1.1 mL/kg per min (+12.7% ± 13%; p<0.02) and VO₂/HR improved by 1.5 ± 1.4 mL O₂ per heartbeat (p <0.05). ANA-VO₂ was reduced by >50% in patients on AAs (from 20.2 ± 10 mL/kg at day 0 to 10.9 ± 5 mL/kg at day 30; p <0.02). These variables did not significantly change for patients who received placebo. In conclusion, the study showed that oral AA supplementation, in conjunction with standard pharmacologic therapy, appears to increase exercise capacity by improving circulatory function, muscle oxygen consumption, and aerobic production of energy in elderly outpatients with CHF.