Objective: To characterize the immunodeficiency in ataxia- telangiectasia (A - T) and to determine whether the immunodeficiency is progressive and associate d with increased susceptibility to infections. Study design: Records of 100 cons ecutive patients with A- T from the Johns Hopkins Ataxia- Telangiectasia Clini cal Center (ATCC) were reviewed. Results: Immunoglobulin (Ig) deficiencies are c ommon, affecting IgG4 in 65% of patients, IgA in 63% , IgG2 in 48% , IgE in 23% , and IgG in 18% . Lymphopenia affected 71% of patients, with reduced B - lymphocyte number in 75% , CD4 T lymphocytes in 69% , and CD8 T lymphocytes in 51% . There was no trend for increased frequency or severity of immune abno rmalities with age. Recurrent upper and lower respiratory tract infections were frequent: otitismedia in 46% of patients, sinusitis in 27% , bronchitis in 19 % , and pneumonia in 15% . Sepsis occurred in 5 patients, in 2 patients concur rent with cancer chemotherapy. Warts affected 17% of patients, herpes simplex 8% , molluscum contagiosum 5% , candidal esophagitis 3% , and herpes zoster 2 % . Uncomplicated varicella infection occurred in 44% of patients; 2 patients had more than one clinical episode. No patient had Pneumocystis jerovici pneumo nia or a complication of live viral vaccine. Conclusions: In spite of the high p revalence of laboratory immunologic abnormalities, systemic bacterial, severe vi ral, and opportunistic infec tions are uncommon in AT. Cross- sectional analysis suggests that the immune defect is rarely progressive. Objective: To characterize the immunodeficiency in ataxia- telangiectasia (A - T) and to determine whether the immunodeficiency is progressive and associate d with increased susceptibility to infections. Study design: Records of 100 conscutives with A- T from the Johns Hopkins Ataxia IgGI in 63%, IgG2 in 48%, IgE in 23%, and IgG in 18% Lymphopenia affected 71% of patients, with reduced B - lymphocyte number in 75%, CD4 T lymphocytes in 69%, and CD8 T lymphocytes in 51%. There was no trend for increased frequency or severity of immune abnomalities with age. upper and lower respiratory tract infections were frequent: otitismedia in 46% of patients, sinusitis in 27%, bronchitis in 19%, and pneumonia in 15%. Sepsis occurred in 5 patients, in 2 patients concur rent with cancer chemotherapy. % of pat Patients with herpes simplex 8%, molluscum contagiosum 5%, candidal esophagitis 3%, and herpes zoster 2%. Uncomplicated varicella experienced occurred in 44% of patients; 2 patients had more than one clinical episode. No patient had Pneumocystis jerovici pneumo nia or a complication of live viral vaccine. Conclusions: In spite of the high p revalence of laboratory immunologic abnormalities, systemic bacterial, severe viral, and opportunistic infec tions are uncommon in AT. Cross-sectional analysis suggests that the immune defect is rarely progressive.