Anaerobic threshold (AT), determined by cardiopulmonary exercise testing (CPET), is a well-documented measure of pre-operative fitness, although its reliability in patient populations is uncertain. Our aim was to assess the reliability of AT measurement in patients with abdominal aortic aneurysms. Eighteen patients were recruited. CPET was performed four times over a 6-week period. We examined shifts in the mean AT to evaluate systematic bias with random measurement error assessed using typical within-patient error and intraclass correlation coefficient (ICC, 3,1) statistics. There was no significant or clinically substantial change in mean AT across the tests (p = 0.68). The typical within-patient error expressed as a percentage coefficient of variation was 10% (95% CI, 8–13%), with an ICC of 0.74 (95% CI, 0.55–0.89). We consider the reliability of the AT to be acceptable, supporting its clinical validity and utility as an objective marker of pre-operative fitness in this population.