Two male patients, aged 46 and 48, presented with muscle pain on exercise, generalized or limited to the upper extremities. Physical signs were limited to mild muscle wasting in one case. There was no myoglobinuria. Electromyography was normal in one case, and showed some polyphasic units in the other. An ischaemic lactate test was normal in the one case in which it was performed. Serum creatine kinase activity was normal or only mildly elevated. Histochemical and ultrastructural study of muscle showed tubular aggregates in type II fibers in both cases, and otherwise gave normal results. Symptoms were unchanged with a follow-up of 2 and 16 years, respectively. Tubular aggregates are found in a number of unrelated clinical and experimental conditions, which are reviewed. In these cases and in 3 similar patients, previously reported, tubular aggregates seem to be the hallmark of hitherto unrecognized metabolic disorders.