Background: To maximise the benefits obtainable from exercise-based cardiac rehabilitation, an evaluation of physical fitness using reliable, clinically relevant tests is strongly recommended. Recently, objective tests of physical fitness have been implemented in the SWEDEHEART register. The reliability of these tests has, however, not been examined for patients with acute coronary syndrome. Aims: The aim of this study was to assess the test-retest reliability and responsiveness to change of the symptom-limited bicycle ergometer test, the dynamic unilateral heel-lift test and the unilateral shoulder-flexion test in patients with acute coronary syndrome. Methods: In a longitudinal study design, a total of 40 patients (mean age 63.8 9.5 years, five women), with ACS, aged amp;lt; 75 years, were included at a university hospital in Sweden. The intra-class correlation coefficient (ICC) with a 95% confidence interval, standard error of measurement (SEM) and responsiveness in terms of the minimal detectable change were calculated. Results: Excellent reliability was found, showing ICC values of 0.98 (0.96-0.99), SEM 4.71 for the bicycle ergometer test, ICC 0.87 (0.75-0.93), SEM 4.62 for the shoulder-flexion test and ICC 0.84 (0.71-0.91), SEM 2.24 for the heel-lift test. The minimal detectable change was 13 W, 13 and 6 repetitions for the bicycle ergometer test, shoulder-flexion and heel-lift tests respectively. Conclusions: The test-retest reliability of clinical tests evaluating physical fitness in patients with acute coronary syndrome included in the SWEDEHEART register was excellent. This makes the future comparison and evaluation of treatment effects in large unselected clinical populations of acute coronary syndrome possible.