Muscle tissue is by far the most abundant cellular tissue in the body and is relatively uniform as to cellular composition. Profound biochemical changes in muscle are known to accompany electrolyte disorders, circulatory disturbances, nutritional deficiencies, and physical exercise. In addition, morphological changes occur in certain muscle disorders and in generalized diseases. Biochemical and morphological studies of muscle tissue in man under various physiological and pathological conditions have proved to be of great value in clinical research and in diagnosis of various diseases. More extensive studies of normal and pathological muscle metabolism require a simple method for muscle sampling. Percutaneous needle biopsy was introduced by Duchenne as early as in 1868 (28) for the investigation of patients with muscular dystrophy. The method seems, however, to have been little used during the following 90 years, until 1957, when Reiffel & Stone (50) suggested that the percutaneous needle biopsy be used for the study of muscle electrolytes in man by neutron activation analysis. In 1960 a percutaneous muscle biopsy technique was developed, using a new biopsy needle (3). It consists of a sharp-tipped hollow outer needle with a small opening (‘window’) near the tip. A cylinder with a sharp edge fits tightly into the needle (Fig. 1). The needle is inserted through a small incision made in locally anaesthetized skin into the muscle (in most cases the vastus lateralis muscle, but other muscles have also been used), and one or more pieces of tissue bulging into the ‘window’are punched out with the sharp cylinder. This method of sampling