From among 115 patients who were treated by intraoperative radiation therapy (IORT), 94 patients were classified according to the histologic findings. The control 127 patients who were treated by operation alone during the same period were also histologically classified, and survivals were compared between the two groups. No difference in the survivals of patients with stage I was observed for the two groups. On the other hand, the survivals of patients with stages II through IV who were treated by IORT increased by nearly 10% to 20% at 5 years. A comparative study was also performed on the survivals between patients treated by IORT and those treated by operation alone according to the presence or absence of the serosal invasion and the grade of the lymph node metastasis. The number of patients treated by IORT in whom the serosal invasion and the lymph node metastasis were histologically examined was 57. The 171 control patients who underwent operation alone were examined histologically in the same fashion, and survivals for the two groups were compared. IORT did not afford any benefit if the lymph node metastasis was limited within n1 group or serosal invasion was not found. On the other hand, the 5-year survival rates for patients who were treated by IORT increased by nearly 10% when the serosal invasion was observed and by nearly 18% when n2 and n3 lymph node metastases were found.