Twelve patients with histologically defined mitochondrial myopathy are described. There were 9 males and 3 females. The age of onset ranged from birth to 35 years with a median of 14 years. The most common clinical picture was that of ophthalmoplegia, ptosis and muscle weakness found in 10 patients. One presented with exercise intolerance due to muscular aches and pains, and the other besides his muscular weakness had mental retardation and an aggressive behavior. The clinical presentation and differential diagnosis of these patients are discussed.