Ataxia-Telangiectasia (A-T) is an autosomal recessive disease that coexists with progressive cerebellar ataxia, immunodeficiency, sinopulmonary infections and skin disorders, including oculocutaneous telangiectasiae, cancer susceptibility, radiosensitivity and early ageing. A-T is caused by mutations of the ATM gene. Laboratory findings include elevated alphaprotein, cerebellar atrophy on MRI, translocations involving chromosomes 7 and 14, absence or dysfunction of the ATM protein and radiosensitivity in CSA. According to recent reports, increased radiosensitivity together with ATM protein absence confirm the diagnosis of A-T. The last step in diagnosis is searching for mutation, which is time-consuming and expensive. As the ATM protein is very large, it influences many processes in organism, and consequently impairs functioning of different organs. Direct and simple guidelines are required for diagnosis and care of A-T patients.