Muscle proteins were extracted in various sodium dodecyl sulfate buffers from 6 patients with myofibrillar myopathy (MFM) and previously identified with mutations in the desmin gene (desmin myopathy; DesM), 6 with MFM without mutations, and 14 disease controls to search for alterations in biochemistry and solubility of mutated desmin filaments. In the 1% posthigh-speed pellet fraction, desmin was detected with immunoblots only in DesM and not the other MFM. We conclude that mutant desmin forms insoluble aggregates that are specific for the DesM and can be detected with Western blots.