Soft tissue sarcomas (STS) are characterized by deregulated proliferation. Ki-67 is a cell cycle antigen which may be elevated in proliferative states. We analysed Ki-67 expression in fixed and embedded tissues from STS in order to examine associations between proliferation, primary tumour characteristics, and metastasis. One hundred and eighty-two adult patients with trunk wall or extremity STS were treated at our institution between 1980 and 1992 (35 developed local recurrence and 56 developed metastases). Median follow-up time for survivors was 6 years (1-13). We used a semiquantitative score to the assess percentage of Ki-67-positive cells: < or = 10% (n = 86), > 10-25% (n = 57), > 25-50% (n = 30), > 50-75% (n = 7), > 75-100% (n = 2). Increasing Ki-67 expression correlated positively with tumour size, malignancy grade, necrosis, vascular invasion, S-phase fraction, and metastasis. A Ki-67 index Ki-D < or = 10% (n = 86) and > 10% (n = 96) defined two groups who had 84% and 56% 3-year metastasis-free survival (p = 0.0001), respectively. Tumours with Ki-D > 10 were typically large, high grade, necrotic, DNA aneuploid, and had intravascular invasion and a higher S-phase fraction. Ki-67 expression may be helpful in predicting survival of patients with soft tissue sarcomas.