Six cases of Ophthalmoplegia Plus (OP) have been studied: all the patients had palpebral ptosis, ophthalmoparesis and descending myopathy. Hypoacusis, cardiac conduction impairment, small stature, mental deficit and vestibulo-cerebellar dysfunction were present only in some cases. EMG showed a severe slowing of motor and sensory conduction velocity in one patient. Polyphasic and long duration MU action potentials, which are indicative of a neurogenic myopathy, were found in all cases. Every muscle biopsy showed many "ragged red fibers" which, in EM, appeared to contain abnormal mitochondria. According to the literature and our data, OP appears to be a multisystemic disorder with severe muscle mitochondrial abnormalities, but it is not certain whether it must be considered a syndrome with multiple etiological factors or a single nosological entity with different possibilities of clinical manifestations.