Background: Progression to symptomatic heart failure is a complication of type 2 diabetes; heart failure onset in this setting is commonly preceded by deterioration in exercise capacity. Objectives: The study sought to determine whether AT-001, a highly selective aldose reductase inhibitor, can stabilize exercise capacity among individuals with diabetic cardiomyopathy (DbCM) and reduced peak oxygen uptake (Vo₂). Methods: A total of 691 individuals with DbCM meeting inclusion and exclusion criteria were randomized to receive placebo or ascending doses of AT-001 twice daily. Stratification at inclusion included region of enrollment, cardiopulmonary exercise test results, and use of sodium-glucose cotransporter 2 inhibitors or glucagon-like peptide-1 receptor agonists. The primary endpoint was proportional change in peak Vo₂ from baseline to 15 months. Subgroup analyses included measures of disease severity and stratification variables. Results: The mean age was 67.5 ± 7.2 years, and 50.4% of participants were women. By 15 months, peak Vo2 fell in the placebo-treated patients by –0.31 mL/kg/min (P = 0.005 compared to baseline), whereas in those receiving high-dose AT-001, peak Vo₂ fell by –0.01 mL/kg/min (P = 0.21); the difference in peak Vo₂ between placebo and high-dose AT-001 was 0.30 (P = 0.19). In prespecified subgroup analyses among those not receiving sodium-glucose cotransporter 2 inhibitors or glucagon-like peptide-1 receptor agonists at baseline, the difference between peak Vo₂ in placebo vs high-dose AT-001 at 15 months was 0.62 mL/kg/min (P = 0.04; interaction P = 0.10). Conclusions: Among individuals with DbCM and impaired exercise capacity, treatment with AT-001 for 15 months did not result in significantly better exercise capacity compared with placebo.