A substantial proportion of patients with primary immunodeficiency diseases develop tumors, particularly those of lymphoreticular system caused by Epstein-Barr virus (EBV). Primary immunodeficiency renders patients susceptible to EBV by reducing immune reactions and surveillance abilities against the virus or inducing overreaction of the responding cells to the antigens. Recent progress in molecular biology has unraveled the genes responsible for several types of primary immunodeficiency diseases. The cloning of the ATM gene demonstrated that the mutations in this gene were observed in the members of all the families affected with ataxia telangiectasia (AT), indicating the crucial role of this gene in the pathogenesis of AT. The protein encoded by the ATM gene shows a high sequence homology with several proteins which are presumed to be involved in the regulation of the cell cycle transition. Accumulating evidence indicates that AT-derived cells are sensitive to irradiation due to the abnormalities in p53-dependent cell cycle arrest at G1 phase. Thus, the ATM product may regulate the cell cycle at G1 phase in a p53-dependent manner and the defect of the gene may lead to the accumulation of cells with DNA damages, thereby causing malignant transformation.