Common variable immune deficiency (CVID) is characterized by low immunoglobulin levels and recurrent infections in patients with a period of normal immune function several gears after birth, It is associated with diarrhea, malabsorption, bronchiectasis, and lymphoreticular malignancies, Radiation-induced chromosome instability mag contribute to the high degree of susceptibility to neoplasia. Peripheral blood lymphocyte cultures were obtained from six patients with CVID and the healthy control group matched by age and sex, The groups did not differ in the frequency of spontaneous chromosome aberrations. After exposure to X-ray radiation, mitotic indices were found to be significantly low and incidence of chromosomal alterations were high in the CVID group. We conclude that chromosomes: of cells from patients with CVID are significantly more. radiosensitive than those of controls, Thus these patients must be protected from unnecessary X-ray examinations and in case of radiosensitive tumour, the dose of irradiation should be carefully monitored. (C) 1998 Academic Press.