αB-crystallin (αBC) and heat shock protein 27 (hsp 27) are members of the family of small heat shock proteins (shsps), which exert a role as molecular chaperones by binding unfolded or denatured proteins, thereby suppressing irreversible protein aggregation and consecutive cell damage. The essential role of shsps in human neuromuscular disorders is highlighted by the observation that a mutation of the human αBC gene causes an autosomal dominant 'myofibrillar myopathy' characterized by αBC and desmin accumulation. Furthermore, an aberrant immunostaining of αBC was recently reported in sporadic inclusion body myositis. In the present study we analyzed the expression and localization of αB-crystallin and hsp 27 in various congenital myopathies by means of indirect immunofluorescence, immunogold electron microscopy and Western blotting. We demonstrate an increased immunoreactivity of αBC and hsp 27 in central and minicore lesions as well as in target fibers, which renders both shsps as reliable, but nonspecific, markers for core and target structures. In contrast, Western blotting demonstrated a normal expression level of αBC and hsp 27, which indicates that the increased immunostaining is not the result of an enhanced protein expression. Furthermore, thiocyanate-induced degradation of actin filaments led to a dramatic decrease of hsp 27 immunostaining in core and target lesions, whereas the increased αBC and desmin immunostaining was found to be even more enhanced. The latter findings imply a functional diversity of both shsps with a preferential association of hsp 27 with the actin microfilament system and αBC with the intermyofibrillar desmin cytoskeleton in human skeletal muscle.