The biological potential of soft tissue leiomyosarcoma is difficult to predict using current standard prognostic parameters, and control of systemic disease is challenging with current chemotherapeutic protocols. Additional prognostic markers and alternative treatment options are very much required. Previous studies implicate upregulation of the oncogenic nuclear transcription factor c-Myc with aggressive behavior of many solid tumors. Therefore, this oncoprotein was evaluated as a prognostic marker for overall and metastasis-free survival in leiomyosarcoma. Immunohistochemical stains for c-Myc were performed on 28 cases of leiomyosarcoma occurring in the deep somatic soft tissues. Comparisons of Kaplan-Meier survival curves stratified by c-Myc status and conventional prognostic factors (histological grade, tumor size, and tumor stage) were evaluated using standard univariate statistical methods. A subsequent multivariate survival analysis was carried out according to the Cox proportional hazards regression model adjusting for potential confounding prognostic factors. A total of 15 cases (54%) were positive for nuclear c-Myc expression. Patients with c-Myc-positive tumors had significantly shorter metastasis-free survival intervals compared with those with c-Myc-negative tumors (median, 9 months vs. >94 months; P=0.014). c-Myc positivity also correlated with decreased overall survival (median, 23 months vs. >94 months; P=0.017). Histological grade was the only other prognostic marker predictive of poor outcome in the univariate analyses. In the multivariate survival analysis, only c-Myc status reached statistical significance, suggesting that it is an important and independent predictor of prognosis in leiomyosarcoma. Detection of nuclear c-Myc in leiomyosarcoma predicts decreased overall and metastasis-free survival, independent of standard prognostic variables, tumor size, histological grade, and TNM stage. The expression of this oncoprotein may represent a useful prognostic marker and potential therapeutic target in leiomyosarcoma.