Introduction Leiomyoma with bizarre nuclei (LBN) and cellular leiomyoma (CL) are rare variants of uterine smooth muscle tumors. In diagnostic practice, LBN can be mistaken for leiomyosarcoma (LMS), while CL may mimic low grade endometrial stromal sarcoma (LG ESS). Careful evaluation of morphological features is necessary when making the diagnosis; in some borderline cases, immunohistochemical and molecular examinations may help. Literature data on molecular genetic alterations in LBN and CL is limited, but some of these tumors appear to share certain aberrations with classical leiomyomas (UL) and LMS. Aims The aim of the work is to expand the knowledge about smooth muscle tumors of the uterus, especially LBN and CL, and perform a complex morphological, immunohistochemical (IHC), and molecular evaluation of their features. The individual goals include: 1) confirmation of the hypothesized benign behaviour of LBN, 2) morphological analysis of LBN, 3) more detailed clarification of LBN tumorigenesis with a focus on the FH gene, 4) clarification of CL tumorigenesis, 5) the use of IHC FH antibody as a screening method to identify FH gene mutations, 6) the use of morphological evaluation and results of IHC examination to facilitate differential diagnostic balance between benign and malignant smooth muscle..