In a prospective study of 102 patients with soft tissue sarcoma, the prognostic importance of DNA content and clinicopathologic features was analyzed. Based on DNA flow cytometry, 37 lesions were diploid (normal DNA content) and 65 were nondiploid (abnormal DNA content). The 5-year metastasis-free survival rate of the whole series was 0.59. The survival rate was 0.77 for patients with diploid tumors and 0.48 for those with nondiploid tumors (P = 0.01). Multivariate analysis identified two independent metastatic risk factors: increasing tumor size and nondiploidy. Unexpectedly, high malignancy grade (III-IV) was not found to be an independent risk factor for metastasis. The risk of metastasis was strongly related to the number of risk factors present. Thus, the 5-year survival for the 76 patients with no or one risk factor was 0.69, as compared with 0.30 for the 26 patients with two risk factors (P less than 0.0001). Our study shows that metastatic disease in soft tissue sarcoma is closely related to nondiploidy. A prognostication model based on DNA content and tumor size was found to discriminate between patients with a good and a poor prognosis after surgical treatment. The model can be used to identify patients who should be excluded from trials with adjuvant chemotherapy.