BACKGROUND The effect of isotretinoin on muscle is considered to be uncommon and benign. We analyzed the files of all our patients given isotretinoin over a 5-year period and determined the incidence and gravity of its effect on muscles. METHODS Sixty treatments with isotretinoin were studied. Myalgia and elevated CPK observed at the pretherapeutic consultation and each month or 2 months thereafter were recorded. RESULTS Muscle symptoms were noted in 60 p. 100 of the cases: myalgia in 51 p. 100 and elevated CPK in 41 p. 100. In this group, male sex (H/F = 2.6) and sports activities (70 p. 100) were found more often. In 5 patients, CPK level was 5 times the normal, defining rhabdomyolysis. One patient interrupted treatment because of persistently high CPK levels. CONCLUSIONS Myalgia and elevated CPK signal skeletal muscle involvement. These signs were more frequent in our series than in reports in the literature, probably because we systematically looked for them. Besides use of isotretinoin, one case of viral infection and sports activities, no other explanatory cause could be found. Isotretinoin could have a potentializing effect on other myotoxicity inducers (drugs, infection, fever, muscular exertion...). The benign nature of the muscle effect appears to be validated although there were some patients who had persistent and/or severe signs.