The family of tumors derived from mesenchymal perivascular epithelioid cells (so-called PEComas) includes angiomyolipoma, lymphangioleiomyomatosis, clear cell sugar tumor of the lung, clear cell myomelanocytic tumor of ligamentum teres/falciform ligament, and abdominopelvic sarcoma of perivascular epithelioid cells. These tumors were characterized by coexpression of melanocytic (HMB-45) and muscle markers. MyoD1 transcription factor has crucial role in commitment and differentiation of mesenchymal progenitor cells to myogenic lineage. Antibodies to MyoD1 protein (nuclear immunoreactivity) have been shown highly valuable adjuncts in the diagnosis of rhabdomyosarcomas. To evaluate expression of the transcription factor MyoD1 in PEComas, we performed immunohistochemistry. Monoclonal antibody 5.8A for MyoD1 was used on a series of cases of formalin-fixed, paraffin-embedded angiomyolipoma (n = 19), lymphangioleiomyomatosis (n = 3), clear cell sugar tumor of the lung (n = 1), and abdominopelvic sarcoma of perivascular epithelioid cells (n = 2). All cases showed strong granular immunostaining in the tumor cell cytoplasm with the anti-MyoD1 antibody. Cytoplasmic reactivity was noted in the spindle cells, fat cells, and epithelioid cells. Nuclei were negative in all tumors studied, and a clean background was obtained. Several normal and neoplastic human tissues have also been immunostained for MyoD1 without any positive cytoplasmic staining, with the exception of 2 alveolar soft part sarcomas. Cytoplasmic immunostaining with monoclonal antibody 5.8A for MyoD1 in PEComas may correspond to cross-reactivity with an undetermined cytoplasmic protein. Great caution should be exercised in interpreting the immunostaining results with anti-MyoD1 antibody 5.8A.