The case is described of a 36 year old man with exercise-induced pain, stiffness, and tubular aggregation in skeletal muscle. Persistent CPK elevation and a normal forearm ischaemic exercise test were demonstrated. Electromyography was normal. Muscle biopsy showed aggregates of tubular structures in most fibres by histochemical and ultrastructural analysis. A relationship of this syndrome to known defects in muscle energy metabolism is postulated.