BACKGROUND: Methotrexate is a cytostatic drug belonging to the group of antimetabolites; it is an antagonist of folic acid with an intensive application in gynecology practice. METHODS: The frequency of micronuclei (MN) in peripheral blood lymphocytes of 30 patients with myoma uteri was analyzed before and after intratumoral application of methotrexate (MTX) in total dose from 50 to 115 mg. Analysis of micronuclei was performed by the application of cytokinesis-block technique (CB). RESULTS: Average frequency of MN in lymphocytes of patients before the therapy was 4.6±0.4 MN/1000 analyzed cells. After the completion of therapy with six separate doses during six consecutive menstrual average frequency of MN increased 1.5 times (7.0±0.6) in comparison to control frequency before the therapy. Statistically significant difference (p<0.001) was established by the Student t test. CONCLUSION: Methotrexate intratumoral treatment of the myoma uteri significantly increased micronuclei in peripheral blood lymphocytes