ABSTRACT: BACKGROUND: The increasing age of coronary artery disease (CAD) patients and the occurrence of sarcopenia in the elderly population accompanied by 'fear of moving' and hospitalization in these patients often results in a substantial loss of skeletal muscle mass and muscle strength. Cardiac rehabilitation can improve exercise tolerance and muscle strength in CAD patients but less data describe eventual morphological muscular changes possibly by more difficult access to imaging techniques. Therefore the aim of this study is to assess and quantify the reliability and validity of an easy applicable method, the ultrasound (US) technique, to measure the diameter of rectus femoris muscle in comparison to the muscle dimensions measured with CT scans. METHODS: 45 older CAD patients without cardiac event during the last 9 months were included in this study. 25 patients were tested twice with ultrasound with a two day interval to assess test-retest reliability and 20 patients were tested twice (once with US and once with CT) on the same day to assess the validity of the US technique compared to CT as the gold standard. Isometric and isokinetic muscle testing was performed to test potential zero-order correlations between muscle diameter, muscle volume and muscle force. RESULTS: An intraclass correlation coefficient (ICC) of 0.97 ((95%CL: 0.92 - 0.99) was found for the test-retest reliability of US and the ICC computed between US and CT was 0.92 (95%CL: 0.81 - 0.97). The absolute difference between both techniques was 0.01 +/- 0.12 cm (p = 0.66) resulting in a typical percentage error of 4.4%. Significant zero-order correlations were found between local muscle volume and muscle diameter assessed with CT (r = 0.67, p = 0.001) and assessed with US (r = 0.49, p < 0.05). Muscle strength parameters were also significantly correlated with muscle diameter assessed with both techniques (range r = 0.45-r = 0.61, p < 0.05). CONCLUSIONS: Ultrasound imaging can be used as a valid and reliable measurement tool to assess the rectus femoris muscle diameter in older CAD patients.