A retrospective review to elucidate the rationale of preoperative irradiation was made on 18 carcinomas of the head of the pancreas area. After 50 Gy/25 fractions of 10 MV X-ray was given, all 18 tumors decreased from 3.3 +/- 0.8 cm to 2.0 +/- 0.7 cm. At the surgical operation, 16 patients (89%) received pancreatic resection, without operative death. Histologically, in 13 of these 16 cases, the population of severely degenerative cancer cells (SDCC) was more than 1/3 of all cancer cells, and SDCCs were likely to locate at the periphery (advancing point of carcinoma). These histological patterns were considered as favorable to improve the operative curability. However, in the remaining three cases, the SDCC population was less than 1/3. In addition, nonaffected (i.e., viable) cancer cells were detected at the periphery, and this is an extremely adverse condition for subsequent surgery. Chronic pancreatitis in the noncancerous area was present in these three cases, but not in the 13 cases. Therefore, the most significant factor that predicts the radioresistance especially at the periphery of the carcinoma was considered to be coexisting chronic pancreatitis.