Background <p>Cardiopulmonary exercise testing (CPET) is the gold standard for assessing cardiopulmonary function, using parameters such as anaerobic threshold (AT), respiratory compensation point (RCP), and peak oxygen uptake (peak VO₂). The time interval from AT to RCP (RCP–AT time) represents the period during which the kidneys regulate bicarbonate reabsorption and buffer lactate to maintain homeostasis. We defined this interval as anaerobic endurance time (AET) and investigated its association with renal function and exercise capacity in outpatients with cardiovascular disease.</p> Methods <p>We retrospectively analyzed 46 patients who initiated outpatient cardiac rehabilitation between June 2020 and October 2024 with available baseline data and 36 patients who completed 1-year follow-up. CPET parameters (AT, RCP, AET, and peak VO₂), physical function (grip strength, knee extension strength, and skeletal muscle index), and mental function were evaluated. Correlation and multiple regression analyses were performed.</p> Results <p>AET significantly correlated with estimated glomerular filtration rate (eGFR) at baseline and 1-year follow-up. Multiple regression analyses identified eGFR as an independent AET determinant at both time points. Similarly, AET was identified as an independent determinant of peak VO₂ at baseline and follow-up.</p> Conclusions <p>AET may serve as a renal function surrogate marker during exercise and is associated with exercise capacity. These associations persisted after 1&#xa0;year of outpatient cardiac rehabilitation, suggesting the long-term clinical relevance of AET as an indicator.</p>