Uterine sarcomas are unusual neoplasms of the female genital tract whose molecular etiology is largely unknown. We examined 20 leiomyomas as well as 23 uterine leiomyosarcomas for the presence of mutations in the Ki-ras and p53 genes, and overexpression of the MDM2 gene. Codons 12, 13, and 61 from the Ki-ras gene were characterized for the presence of mutations by restriction enzyme polymorphisms using mismatched primers and nucleic acid sequencing as appropriate. Activated Ki-ras genes were identified in 3/20 leiomyomas and 0/23 leiomyosarcomas. The p53 gene was analyzed by SSCP, nucleic acid sequencing, and immunohistochemical staining. None of 20 leiomyomas and 6/23 leiomyosarcomas exhibited p53 abnormalities. The SSCP/sequencing results did not consistently correlate with the IHC staining. MDM2 overexpression occurred in 0/20 leiomyomas and 3/23 sarcomas. Clinical correlation suggested that tumors with p53 mutations have a higher histologic grade or stage at presentation. We conclude that leiomyomas and leiomyosarcomas have different patterns of molecular alterations and are separate biologic entities. In addition, p53 and MDM2 overexpression may play a role in the development of a subset of leiomyosarcomas.