The authors investigated the effects of radiation therapy on the immune system by studying lymphocyte subsets and other parameters in 32 patients undergoing radiation therapy for solid cancer. With monoclonal antibody techniques, we studied both T- and B-lymphocytes; cell suspensions were analyzed by means of a Facs Spectrum III Ortho (Ortho-Diagnostic) unit. The first control was performed right after the beginning of radiotherapy, when the dose to the patients was 50 Gy or higher. The second control was performed at 40 Gy because all patients received this dose. 30% of the patients exhibited lymphopenia from the beginning of the study; at 40 Gy the number of T-lymphocytes was low and helper/suppressor ratio was altered. A variable response of B-cells was observed, although all patients exhibited restoration of normal values at 6 months. Four patients only suffered from side-effects: a patient with tongue cancer presented oral mycosis, and a woman--treated for breast cancer--presented vaginal mycosis. Two cases of cystitis were also observed, after 18 Gy, in patients with uterine carcinoma undergoing pelvic irradiation. Disease progression was observed in 2 patients with head and neck cancer, while 3 patients died from lung cancer progression. Another one, with head and neck cancer, died because of heart failure.