ABSTRACT Introduction We investigated the effect of high‐intensity exercise on plasma creatine kinase (CK) in patients with muscular dystrophies. Methods Fourteen patients with Becker (BMD), facioscapulohumeral (FSHD), or limb‐girdle type 2 (LGMD2) muscular dystrophy, and 8 healthy subjects performed 5 cycling tests: an incremental max test, and tests at 65%, 75%, 85%, and 95% of maximal oxygen uptake (VO2max). Heart rate and oxygen consumption were measured during the tests, and plasma CK was measured before, immediately after, and 24 hours after exercise. Results All subjects were able to perform high‐intensity exercise at the different levels. In patients with LGMD2 and FSHD, CK normalized 24 hours after exercise compared with the pre‐exercise value, whereas those with BMD and healthy controls had elevated CK values 24 hours after exercise. Conclusions The findings suggest that high‐intensity exercise is generally well tolerated in patients with LGMD2 and FSHD, whereas those with BMD may be more prone to exercise‐induced damage. Muscle Nerve 48: 897–901, 2013