Ataxia Telangiectasia (AT) is named after the two key clinical features that characterize its classical phenotype, namely a progressive cerebellar gait disorder (ataxia) and vascular anomalies (telangiectasias) visible in the conjunctivae and skin. AT is an autosomal recessively inherited disorder, caused by mutations in the ATM gene that encodes the ATM protein. While the ataxia is subject of many publications, the telangiectasias are under emphasised. We here describe the observation that the absence or presence of ATM protein and the level of residual ATM kinase activity are related to the occurrence of telangiectasias and describe the clinical consequences of these vascular malformations. Finally, we hypothesize that ATM dysfunction dysregulates angiogenesis.