Studies have shown a 15-30% frequency of microsatellite instability in endometrial cancer. In addition, we found a 21% frequency of microsatellite instability in endometrial cancer. Our aim was to investigate the presence of microsatellite instability and loss of heterozygosity in uterine sarcomas. The records of 69 women referred to Kalafong Academic and Pretoria Academic Hospital with a primary diagnosis of uterine sarcoma were reviewed. At histological review of 43 cases with a primary diagnosis of leiomyosarcoma, diagnosis of mitotically active leiomyoma was made in 21. Diagnosis of carcinosarcoma was made in 21 cases and endometrial stromal sarcoma in five. In all cases, genomic DNA was extracted from normal myometrium and tumor and analyzed for microsatellite instability and loss of heterozygosity. High-frequency microsatellite instability was absent in leiomyosarcoma, endometrial stromal sarcoma, and mitotically active leiomyomas and was observed in 1 (5%) carcinosarcoma. Loss of heterozygosity for chromosome 11 was present in 8/48 (17%) of uterine sarcomas, equally distributed between leiomyosarcomas (4/22 = 18%) and carcinosarcomas (4/21 = 19%). There was no loss of alleles in endometrial stromal sarcoma nor mitotically active leiomyomas. In conclusion, it is suggested that tumor suppressor genes may play a role in the tumorigenesis of uterine mesenchymal cells, whereas mismatch repair genes contribute to the carcinogenesis of endometrial cancer.