Serum testosterone level (STL) was examined in 25 male patients with cardiac or gastric carcinoma before and after operation and 28 male patients with benign diseases as control. The preoperative STL of patients with carcinoma was lower than that of the control (P less than 0.001). Postoperatively STL of these patients rose to the level of control group. When cancer recurred, STL again dropped to low level. STL was lower when cancer had invaded beyond the serosa than that limited to the musculature. Therefore, STL may be a good parameter for diagnosis and prognosis in patients with cancer of the stomach and gastric cardia.