The microfilament-associated protein vinculin is a major constituent of muscle tissue localized in costameres and Z-discs of the sarcomeric apparatus, where it is thought to play a pivotal role in the alignment of sarcomeric myofibrils and the transduction of mechanical force between the internal contractile machinery and the extracellular environment. In order to investigate whether anti-vinculin antibodies are helpful in confirming the commitment of rhabdomyosarcomas to the myogenic pathway, we studied immunohistochemically the expression pattern of vinculin in a series of 7 human rhabdomyosarcomas including those of embryonal, botryoid, and pleomorphic subtypes. Using monoclonal antibody from clone hVIN-1 by APAAP techniques on formalin-fixed, paraffin-embedded tissue, all but one tumor, which was a primitive embryonal rhabdomyosarcoma, demonstrated a significant positive vinculin staining. Vinculin expression was most prominent in differentiated tumors with a focal staining pattern showing a high degree of correlation with rhabdomyoblasts, whereas a diffuse staining was observed in areas in which small, poorly differentiated tumor cells alone were present. Since vinculin immunoreactivity could also be demonstrated in cases of leiomyosarcoma, the positive immunohistochemical detection of vinculin was not exclusively restricted to mesenchymal tumors derived from sarcomeric muscle tissue. Immunodetectable amounts of nebulin could be revealed only in two embryonal rhabdomyosarcomas. Our results suggested that the positive identification of rhabdomyosarcoma achieved by using antibodies against vinculin in addition to other known myogenic markers may be particularly useful in the differential diagnosis of anaplastic, poorly differentiated sarcomas.