Cytogenetic analysis has defined specific translocations associated with two of the most common small round cell tumors of childhood, t(11;22) in Ewing's sarcoma and t(2;13) in alveolar rhabdomyosarcoma. We and others have previously demonstrated the diagnostic utility of a reverse transcriptase polymerase chain reaction (RT-PCR) assay for the detection of the t(11;22) encoded EWS/FLI-1 chimeric message in Ewing's sarcoma. More recently, we have cloned the t(2;13)(q35;q14) translocation and have shown that it results in the fusion of the PAX3 gene on chromosome 2 to FKHR, a novel member of the fork-head family of transcription factors on chromosome 13. To define the morphological spectrum of childhood sarcomas that express the t(2;13) encoded PAX3/FKHR chimeric message, we have performed RT-PCR analysis on samples from 44 primary pediatric sarcomas and 8 sarcoma cell lines. PAX3/FKHR chimeric messages were detected in 24 of 27 alveolar, 2 of 12 embryonal, and 0 of 1 pleomorphic rhabdomyosarcoma and in 1 of 2 ectomesenchymomas. In contrast, none of 8 Ewing's sarcomas or 2 undifferentiated sarcomas expressed this message. Chimeric transcripts were detected in all cases with cytogenetic evidence of the (2;13) translocation, and in each case the chimeric PAX3/FKHR message had the identical junction sequence, suggesting that genomic chromosome breaks were clustered in a single intron in both genes. By combining the PAX3/FKHR RT-PCR assay with primers for detection of the Ewing's sarcoma t(11;22) encoded EWS/FLI-1 chimeric transcript, we have developed a multiplex RT-PCR reaction that allows the rapid and accurate identification of either translocation in a biopsy sample.