Endometrial stromal neoplasms include a heterogenous group of tumors with different clinical behavior and response to treatment. Cytogenetic study of such tumors has been scanty. The aim of this study was to analyze and compare the chromosome composition in low-grade and high-grade endometrial stromal sarcoma using the technique of chromosome in situ hybridization. Eight cases of low-grade stromal sarcoma and three cases of high-grade stromal sarcoma were studied. Biotinylated DNA probes specific for the regions of chromosomes X, 11, 12, and 17 were used on formalin-fixed paraffin embedded material from these tumors. The in situ hybridization signals were visualized by immunoperoxidase technique. Four of the eight low-grade stromal sarcomas retained normal disomy of the chromosomes studied. The other four low-grade sarcomas showed a gain of from one to three chromosomes, whereas all of the three high-grade sarcomas showed polysomies in all of the four chromosomes being studied. No loss of chromosomes was detected. One case of high-grade sarcoma contained coexisting areas of low-grade sarcoma. Although aneusomy was found in the high-grade portion, disomy was noted in the low-grade areas. In the stromal sarcomas studied, there was no definite correlation between the presence of chromosome polysomies and the clinical progress of the tumors. This is the first interphase cytogenetic study of uterine stromal sarcoma, and the results support the concept that complex numerical chromosome abnormalities evolve during anaplastic transformation of endometrial stromal sarcoma.