PURPOSE: To evaluate the clinical outcomes and prognostic factors in retroperitoneal soft tissue sarcomas treated by postoperative radiotherapy. MATERIALS AND METHODS: The records of 23 patients with retroperitoneal soft tissue sarcomas, who underwent postoperative radiotherapy between 1985 and 2003, were analyzed. The median follow-up period was 77 months (range, 8~240 months). A total of 21 patients presented with primary disease, and two patients presented with recurrent disease. Liposarcomas and leiomyosarcomas represented 78% of the diagnosed tumor cases. Moreover, 17 cases were of high grade (grade 2 or 3). The median tumor size was 13 cm (range, 3~50 cm). Complete excision was achieved in 65% of patients. The median radiation dose was 50.4 Gy (range, 45.0 to 59.4 Gy), with conventional fractionation. RESULTS: The 5-year overall, local recurrence-free, and distant metastasis-free survival rates were 68%, 58%, and 71%, respectively. Eleven patients experienced local recurrence, while 9 patients experienced distant metastasis. The most common site for distant metastasis was the liver. A univariate analysis revealed that adjacent organ invasion and age (> 60 years) as the significant risk factors contributing to the prediction of poor overall survival. Moreover, multivariate analyses indicated that adjacent organ invasion remained significantly associated with a higher risk of death. In addition, patient age (> 60 years) was the other identified risk factor for local recurrence by univariate and multivariate analyses. Except for one case of grade 3 diarrhea, no patient suffered grade 3 or higher complications. CONCLUSION: Our results were comparable to previous reports in that adjacent organ invasion and patient age (> 60 years) were significant predictors of poor survival and tumor recurrence, respectively.