Leiomyosarcoma of the somatic soft tissue is a rare malignant mesenchymal neoplasm that metastasizes to other organs in a subset of cases. Much remains to be learned about the mechanisms underlying the development of aggressive behavior of this tumor. It has been difficult to predict the clinical behavior of leiomyosarcomas using the morphology-based grading system, even though tumor size and histological grade have correlated with biologic behavior in some studies. In this study we analyzed the gene expression patterns of 35 samples of mesenchymal origin, including 11 cases of leiomyosarcomas of different histological grades arising in soft tissue and the retroperitoneum, using the Affymetrix U133a chips, which contain more than 22,000 genes and expression sequence tags (ESTs). We identified a set of genes whose expression was commonly altered in all leiomyosarcoma samples. In addition, we identified specific gene expression patterns in several subsets of the tumor. We used these alterations of gene expression to subclassify the leiomyosarcomas into 3 groups. Interestingly, the grouping of these samples correlated well with tumor differentiation and clinical aggressiveness. The analysis identified 92 genes that distinguish low-grade, well-differentiated leiomyosarcomas from less well-differentiated, high-grade, and metastatic leiomyosarcoma. Thesse alterations of gene expression appear to be correlated with the clinical behavior and histological grade of the tumor. The striking differences in terms of gene expression pattern among leiomyosarcomas of different differentiation status and clinical aggressiveness imply that several genetic abnormalities are responsible for the genesis and progression of this tumor.