We report the first isolation of influenza virus from muscle in a man with myoglobinuria and acute polymyositis. Influenza virus was isolated from cultures of Madin Darby bovine kidney and primary rhesus monkey kidney cells inoculated with muscle homogenates in the presence of trypsin; the virus was identified by neutralization and hemagglutination inhibition studies using influenza B/Lee antiserum. Viral plaque assay was performed with Madin Darby canine cells. Viral antigen was also detected by specific immunofluorescence in muscle, and myxovirus-like particles were seen in subsarcolemmal vacuoles by electronmicroscopy. The pathologic findings were similar to those of childhood dermatomyositis, except for a large proportion of necrotic muscle fibers. The evidence suggests that the pathogenesis of influenzal polymyositis in this patient involved direct viral infection of muscle.