Nonrandom cytogenetic abnormalities of chromosomes 6, 7, and 17 have been reported within low-grade endometrial stromal sarcomas (LGESSs), and among these abnormalities, the t(7;17)(p15;q21) is the most common aberration described. Previously we had shown that this translocation joins 2 genes, JAZF1 and JJAZ1, located on chromosomes 7 and 17, respectively. To determine the frequency of the t(7;17), we analyzed 4 stromal nodules and 24 LGESS by both reverse transcriptase-polymerase chain reaction and fluorescence in situ hybridization (FISH). In addition, we examined 4 cases of highly cellular leiomyoma, a benign morphologic mimic of LGESS. Overall, evidence for the JAZF1-JJAZ1 fusion was found in 60% of endometrial stromal neoplasms analyzed (8/16 ESS and 4/4 stromal nodules). One LGESS demonstrated only rearrangement of 7p15 by FISH analysis and karyotypic analysis of this case showed t(6;7)(p21;p15). The fusion was not detected in any highly cellular leiomyomas. Our data suggest that the JAZF1-JJAZ1 fusion is a frequent, although nonuniform, feature of endometrial stromal neoplasia, irrespective of benign versus malignant classification and smooth muscle differentiation. In addition, the detection of the fusion by reverse transcriptase-polymerase chain reaction or FISH for JJAZ1 at 7p15 may be diagnostically useful.