Serum testosterone level (STL) was determined in 52 male patients with cardiac carcinoma and in 48 male patients with gastric carcinoma before and after operation, and in 54 normal male subjects and preoperative 40 male patients with benign diseases as control groups. The STL of the patients with cardiac carcinoma or gastric carcinoma was lower than the normal group and the benign diseases group (P less than 0.001, respectively). The postoperative STL of the patients with cardiac or gastric carcinoma was not significantly different from the normal group and the benign diseases group. The STL was lower in patients with recurrence than in the normal group and benign diseases group (P less than 0.001, respectively), and lower in patients with cardiac or gastric carcinoma invading outside the serosa than in those with lesions confined to the musculature (P less than 0.001 and P less than 0.05). Therefore, it appears that the STL may be an important parameter in diagnosis, and in the evaluation of prognosis, postoperative recurrence and resectability of the tumors.