Ewing's sarcomas and malignant peripheral neuroectodermal tumors (MPNTs) show very little evidence of differentiation and lack characteristic morphological features at the light-microscopic level. These malignancies have always presented a significant differential diagnostic challenge to the pathologist. Electron microscopy, immunohistochemical staining for neural antigens such as neuron-specific enolase (NSE), Leu 7, synaptophysin and, more recently, the detection of Mic-2 gene expression have been included in the routine histopathological diagnostic procedure. However, the expression of these antigens is not restricted to this entity. Thus, further modalities are required to prove diagnostic reliability. One consistent feature of the Ewing's sarcoma family is the presence of the reciprocal chromosomal t(11;22)(q24;q12) translocation. Recent cloning of the t(11;22) break point has led to the identification of the genes involved in this translocation. This provides the possibility of molecular genetic detection of the t(11;22) translocation in Ewing's sarcomas and MPNTs. We have established a method using reverse transcription and the polymerase chain reaction (RT-PCR) for the detection of the specific gene fusion transcript caused by the 11;22 translocation. The validity of our approach was proved by analyzing Ewing's tumor cell lines and tissue material obtained from primary biopsies and tumor resections. Molecular genetic detection of the 11;22 translocation by RT-PCR analysis should perhaps be included in the diagnostic work-up of suspected Ewing's sarcoma and MPNT.