Three cases of centronuclear myopathy are reported in a sibship. The disease starts during the late childhood and has a very slow progressive course. Clinical examination reveals a ptosis of the eyelids, a weakness of the facial muscles, a high-arched palate and a diffuse involvement of the lower extremities. The respiratory muscles are involved in the two oldest siblings. Combined morphological studies of the tibialis anterior muscle in one patient demonstrate the following features: 1. the presence of small type I fibres and of large type I or type II fibres; 2. a predominance of type I fibers which represent 86% of the total amount of muscle fibres; 3. central nuclei in about 98% of the muscle fibres; 4. moderate ultrastructural changes. All these features are compared with the relevant data of the literature.