The significance of primary tumor resection in gastric and colorectal cancer patients with liver metastasis (H(+)) was evaluated in terms of operative mortality and survival rate by dividing the materials [293 gastric cancer and 80 colorectal cancer patients (53 colon and 27 rectum) with synchronous liver metastasis] into the following groups: Firstly, with or without peritoneal dissemination (P), secondly, with or without resection of the primary tumor and thirdly, with or without postoperative adjuvant chemotherapy. The following results were obtained: (1) The direct operative death rate of primary tumor resection, excluding death from other causes, showed an absence of statistically significant differences between the P0H(+) and P(+)H(+) gastric and colorectal cancer patients. (2) There was no significance in the prognosis between the primary tumor resection + postoperative chemotherapy group and the non-resectable group in the P(+)H(+) gastric and colorectal cancer patients, revealing no prognostic value of the primary tumor. (3) In the P0H(+) gastric and colorectal cancer patients, the primary tumor resection + postoperative chemotherapy group was significantly more favorable in prognosis than was the primary tumor resection alone group or the non-resectable group, showing the value of primary tumor resection.