The abscopal effect is the radiation response in tissue at a distance from the irradiated site invoked by local irradiation. It is reported that the abscopal effect is observed occasionally in the cases of radiotherapy for malignant lymphoma, malignant melanoma and seminoma. However, pathophysiology and mechanism of the abscopal effect have not been well defined. An aim of this study was to investigate the pathophysiology and mechanism of the abscopal effect in patients with breast carcinoma. Sixty two patients entered this study. Age distribution was from 29 to 84 years old (mean 54.0 years old) and all cases were females. Their stages were as follows: stage II 12 cases, stage IIIa 16 cases, stage IIIb 22 cases and stage IV 12 cases. They were irradiated pre-operatively using less fractionated large dose irradiation. Then, they underwent mastectomy or tumor resection. The abscopal effect on metastatic lymph nodes was observed in 15 out of 42 cases (35.7%) by palpation. The histopathological abscopal effect was noted in 22 out of 42 cases (52.4%). Incidence of the abscopal effect was significantly higher in patients under 55 years old as compared with that in patients over 56 years old (p less than 0.05). The abscopal effect was highly observed in the cases who had the infiltrating lymphocytes around the degenerated cancer cells in the irradiated primary tumor nests (p less than 0.01). The subsets of the infiltrative lymphocytes were analyzed immunohistologically using monoclonal antibodies. It disclosed that the infiltrative lymphocytes were CD8 and CD4 positive lymphocytes. Those findings suggest that abscopal effect was caused by activated cellular immunity in hosts. The five and ten years survival rates of stage IIIa were 71.4% and 71.4% respectively. Stage IIIb showed 62.5% in 5 years survival rate and 54.7% in 10 years survival rate. The survival rate of the cases with the abscopal effect was higher than that of the cases without the abscopal effect, however it was not statistically significant. No complications such as pneumonia was observed.