Ataxia-telangiectasia (A-T) is characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, chronic sinopulmonary disease, a high incidence of malignancy, and immunological deficiencies. The mutated gene, ataxia-telangiectasia mutation (ATM), has been mapped to chromosome 11 (11q22-23). The incidence of A-T is about 1-3 per 40,000-100,000 births. The aim of this study was to evaluate the clinical and immunological features of the patients with A-T. Medical records of 49 patients with A-T were reviewed retrospectively. Demographic features, laboratory findings, genetic analysis, and the clinical data recorded during the follow-up period were reviewed. Medical records of 49 patients (26 female) were included in the study. Mean age at the time of study was 9 years 3 months. Mean follow-up period was 4.5 years. Mean diagnostic delay was 18 months. Consanguinity was found in 85.7% of the patients. Malignancy was present in 8.2% of first-degree and in 34.7% of second-degree relatives. Lymphocytopenia was found in 29 (59.2%) patients. Mean serum alpha-fetoprotein level was 171.6 +/- 138.6ng/dL. Intravenous immunoglobulin replacement therapy was required in 28 (57.1%) patients. Sinopulmonary infections were the most common infections (51%). Fifteen (30.6%) patients became wheelchair dependent. Malignancy occurred in 6 (12.3%) patients. One patient with acute lymphoblastic leukemia had successful bone narrow transplantation. The other 5 patients died during the follow-up period. Mutation in ATM was found in 34 (69.4%) patients. Of these, 7 patients were found to have novel mutations. A-T is difficult to treat and has a poor prognosis because of its multisystem involvement. Management strategies focus on the surveillance, prevention, and treatment of the major causes of morbidity related to A-T.