Percutaneous needle biopsy was carried out in 75 patients with various neurological and muscle diseases (myopathies in 7, lower motor neuron diseases in 22, upper motor neuron diseases in 6, and unlocalized or miscellaneous diseases in 13) and its usefulness as a diagnostic procedure was evaluated. The ages of the subjects ranged from 2 months to 35 years, 77% were younger than 6 years. The materials were examined histologically, histochemically and electron microscopically with the following results. 1. The muscle volume obtained by needle biopsy was sufficient for studies in 64 cases (85%), insufficient in 8 (11%), while in 3 cases no muscle tissue was obtained. 2. Histological staining properties were excellent in all samples examined. In histochemical preparations, staining was judged as good in 61 cases, but poor in 11 due to an inappropriate freezing procedure. 3. In 33 cases, both needle and open biopsies were carried out simultaneously, and the morphological findings of the specimens were compared. There were no significant differences between them histologically as well as histochemically. Electron microscopic examinations also provided identical information in the seven cases studied so far. 4. The method of needle biopsy has many advantages over open biopsy as a routine diagnostic procedure for neuromuscular disorders in infants and young children, but there are a few disadvantages such as the requirement of technical skill and the inappropriateness for histogram construction in some cases.