Thymosin α1 is a cleavage product of prothymosin α. Expression of prothymosin α increases during cell proliferation. Thymosin α1, measurable in plasma, may be an indicator of cell proliferation especially if a cancer is present. In this report we investigated the relation between the clinical behavior of gastric cancer and the plasma thymosin α1 level. Plasma thymosin α1 was measured in 52 gastric cancer patients using a newly developed radioimmunoassay. Twenty-one tumors and lymph nodes were examined for thymosin α1 using immunohistochemistry. The plasma thymosin α1 level was higher in gastric cancer patients than in normal volunteers; and it was higher in patients with lymph node involvement than in those with negative nodes. Immunohistochemical study of thymosin α1 showed positivity in 52% of gastric cancers and 100% of lymph node metastasis. We concluded that a high level of plasma thymosin α1 suggests aggressive behavior of a gastric cancer, such as lymph node involvement.