According to several studies, even the locoregional irradiation of patients with carcinoma can cause a severe and rather alarming cellular immune defect. We thus designed a prospective research in order to study the effect of post-operative irradiation on cellular immunity in patients suffering from breast cancer. In 35 patients with breast cancer who required post-operative irradiation, four blood samples were taken at indicated point times. Nineteen out of 35 patients received post-surgical chemotherapy before irradiation. The total lymphocytes as well as CD4 and CD8 subpopulations were measured by using flow cytometry analysis. The mean T-lymphocyte (Tol) count dropped from 1487.77 to 1227.91 (P = 0.0013) and the CD4+ count from 674.17 to 580.91 (P = 0.0189). The mean value of CD8+ dropped from 421.31 to 314.00 (P = 0.0003). Moreover, a statistically significant difference regarding the pattern of temporal change was observed between a group of patients that received irradiation only and a group that received radiation therapy (RT) with chemotherapy (P-values 0.0015, 0.01 and 0.092 for Tol, CD4+ and CD8+ respectively). The group of patients that received RT only presented a more rapid decrease of Tol concerning the decrease observed in the group that underwent chemotherapy and RT.