Objective — to investigate microsatellite instability in smooth muscle tumors of uncertain malignant potential and to compare the results with clinical and morphological data. Subject and methods. Histological and immunohistochemical studies were conducted in 26 patients aged 30—63 years (mean age, 37 years) with leiomyomatosis; which revealed intravenous leiomyomatosis in 20 cases, metastasizing leiomyoma in 2, disseminated peritoneal leiomyomatosis in 3, and smooth muscle tumor of uncertain malignant potential in 1 case. Microsatellite instability was studied by fragment analysis on a genetic analyzer using a test system of six markers: D10S1146, D10S218, D10S24, D10S1213, D3S1295, and D9S942. Results. Microsatellite repeat changes characteristic of leiomyosarcomas (heterozygosity loss and/or microsatellite instability in at least one locus studied) were found in 6 patients; all were clinically and morphologically diagnosed as having intravenous leiomyomatosis. In 3 of these 6 cases, leiomyomatosis was accompanied by metastases to the lungs and spread to the peritoneum; heart damage was noted in 2 cases. The data analysis did not allow identification of any significant clinical and morphological criteria for this group. Conclusion. Leiomyomatosis is not a transitional form from benign leiomyoma to leiomyosarcoma, as evidenced by the difference in the status of molecular markers. Analysis of molecular genetic changes in DNA from tumor tissue samples cannot categorically clarify the nature of the disease by identifying the signs of genetic instability; however, there is a need for further accumulation of experience in studying tumors of this group and in identifying the possible association with disease prognosis.