Desmoplastic small round cell tumor (DSRCT) is a rare, aggressive malignancy characterized cytogenetically by a unique translocation of chromosomes 11 and 22 [t(11:22)(p13:ql2)], resulting in fusion of the EWS and WT1 genes. The presence of a unique fusion mRNA in DSRCT allows disease detection and diagnosis by reverse transcription polymerase chain reaction (RT-PCR), as previously described in fixed paraffin-embedded material. In this report, EWS WT1 fusion mRNA was detected in ascites from a patient with DSRCT by RT-PCR. RT-PCR results confirmed the diagnoses of DSRCT and of malignant ascites at the molecular level. RT-PCR assays for specific molecular markers, such as EWS-WT1 fusion mRNA, are potentially powerful methods that can complement routine histological, cytological, and/or immunohistologic assays.