A 34 year old man presented with an 8 year history of mild muscle pain and stiffness on exertion especially in the cold. Clinical examination was normal. Apart from a mild persistent leucocytosis, his routine investigations were normal including creatine kinase activity, electromyography and nerve conduction studies. An ischaemic exercise test produced a slow and incomplete rise in lactate. Histological examination showed non-specific myopathic changes in some quadriceps femoris muscle fibres. Investigation of muscle metabolism by spectrofluorometric analysis of muscle enzyme activity and by muscle fibre incubation studies revealed a severe defect in glucose phosphorylation, associated with an electrophoretically abnormal hexokinase. Further metabolic studies suggest that the block in glucose metabolism is by-passed via an enhanced phosphorylation of fructose by the abnormal hexokinase.