Most of the neuromuscular diseases have similar clinical presentation affecting mainly young patients. Clinical observations, serum enzymes, electromyographic studies and muscle biopsy are required for correct diagnosis. The authors show the results of morphological observations of a series of 124 muscle biopsies studied between 1988 and 1992 using conventional paraffin embedded material, frozen sections of muscle tissue stained by several histochemical techniques, electron microscopic observations and, in some cases, morphometric analysis of the smaller diameter of at least 100 random muscle fibres. Neurogenic atrophy was present in 27% of the cases (n = 33), dystrophic muscle was diagnosed in 26%, mitochondrial myopathy in 7% while inflammatory and metabolic myopathies were less frequent. In the group of muscle dystrophy most cases were of Duchenne type showing great variation in fibre size ranging from 10 to 110 microns (mean = 30 to 70 microns). There was also muscle necrosis and fatty changes. Neurogenic biopsies showed fibre atrophy with clustering of nuclei and group atrophy. Electron microscopic observations in cases of Werdnig-Hoffmann showed tiny fibres with less than 5 microns in great dimension and also with redundant basal laminae. Cases of mitochondrial myopathy showed the classical ragged red fibres and many different mitochondrial abnormalities under the electron microscope. Many cases (33%) showed only minor structural abnormalities or end-stage alterations with marked fatty infiltration and fibrosis so that a definitive diagnosis failed to be achieved. The correct selection and handling of muscle biopsies is important for adequate diagnosis in neuromuscular pathology.