Leiomyoma is a common benign tumor of the genital system; diseases and other types of smooth muscle tumors in females is called leiomyosarcoma. The diagnosis of these tumors is based on differentiation of the degree of cellularity and cytological atypia, presence of necrosis and particularly the mitotic index. A treatment procedure is proposed regarding to the diagnosis of leiomyoma and leiomyosarcoma. There is no exact agreement on diagnosis of these tumors and the available methods are biased and dependent on the judgment of the observers. Our aim is to evaluate the leiomyoma and leiomyosarcoma type of tumors using design-based stereological methods. The number of cells showing mitosis within a reference volume was determined to diagnose the type of tumor as leiomyoma or leiomyosarcoma. Numerical density of mitotic figures was 8000-9000/mm3 for leiomyoma, 10.000-11.000/mm3 for leiomyoma with high mitotic activity, and 30.000-31.000/mm3 for leiomyosarcoma. The difference between leiomyoma and leiomyosarcoma was significant (p<0.001). These results show that the proposed method of providing the numerical density of mitosis for the diagnosis of leiomyosarcoma is unbiased, objective, and reliable. Moreover the method is more effective and easier to use than a morphometric method. © 2009 OMU AU rights reserved.