Ataxia-telangiectasia (AT) is an autosomal recessively inherited disease (one case in 40,000 to one case in 100,000 live births) whose principal features are oculocutaneous telangiectasia, progressive cerebellar ataxia, B- and T-cell immunodeficiency with recurrent sinopulmonary infections, sensitivity to ionizing radiation and cancer predisposition. The AT-gene (ATM) was recently identified by positional cloning on chromosome 11q22-23. In this paper the diagnostic, clinical and therapeutic problems of 9 AT-patients treated in our clinic are discussed in context with the current literature. Although all patients had discrete signs of cerebellar ataxia at infancy, there was a significant delay of definitive diagnosis (median 4, range 1.5-6.5). Elevated alpha fetoprotein levels clearly distinguish AT from other ataxias and immunodeficiency syndromes.