Aim To study the content and clinical significance of serum endothelin-1(ET-1)in patient with malignant tumors. Methods The contents of serum ET-1 in 85 patients with malignant tumors,30 patients with benign diseases and 30 normal indivaduals were detected by radioimmunoassay. Results (1)The mean concentrations of serum ET-1 in normal individuals and in non-tumor patients were (46.9± 23.1)μ g/L and (51.1± 30.9)μ g/L,respectively, while in patients with malignant tumors it was (190.1± 135.2～ 382.4± 190.1)μ g/L. (2)There was no significant difference between the mean serum contents of ET-1 in normal individuals and non-tumors pstients(P∧ 0.05),but the level of serum ET-1 in patients with malignant tumors were significantly higher than that in above two groups(P∨ 0.001). (3)When the mean content plus twice standard deviation of serum ET-1 in normal individuals was as a threshold value,that is ,the value was defined as the positive clinical value to diagnose cancer, the positive rates of patients with different malignant tumours were 72％ ～ 100％ . Conclusion Serum ET-1 contents could be used as a better tumor index for maligant tumor screening and accessory diagnosis.