Heat-shock/stress proteins are constitutive and stress-inducible proteins, regulated by a number of factors including developmental processes. The 90-kD heat-shock protein (hsp90) and ubiquitin are up-regulated in regenerating fibers and diseased fibers of Duchenne muscular dystrophy. The aim of the present study was to investigate whether the heat-shock response in regenerating fibers is developmentally regulated or disease-associated. Immunohistochemistry and immunoblot analysis were employed to compare the expression of hsp90 and ubiquitin in normal immature muscle from infants and regenerating fibers in polymyositis and dermatomyositis with the basal expression in normal mature muscle from adults. A significant up-regulation of hsp90 and ubiquitin in regenerating fibers and developing infantile fibers suggests that hsp90 and ubiquitin, during myogenesis, are largely regulated by the activation of developmental mechanisms rather than being primarily disease-related. Modulation of the stress response may promote myogenesis and provide a new therapeutic approach in myopathies.