A clinical, genealogical, EMG and morphological study of the development of pseudohypertrophic myopathy linked with X-chromosome allowed one to distinguish 2 variants: 1) the malignant (Duchenne form) and 2) benign (Baker form). Although both forms have similar clinical symptoms, the Baker form has a later onset, a more favourable development and more preserved motor functions. The activity of fructosodiphosphataldolase and creatinphosphokinase in the blood serum of these patients was significantly lower than in Duchenne' myopathy. In the muscular bioptates of patients with Duchenne's myopathy as the disease progresses there is a gradual smoothening of the diameter of preserved elements at the expense of almost complete disappearance of hypertrophysed filaments. In Baker's myopathy besides expressed processes of muscular sclerosis and lipomatosis there are also hypertrophysed filaments.