Ataxia-Telangiectasia (A-T) is a very rare autosomal recessive disease characterised by cerebellar ataxia, extrapyramidal movements and cancer predisposition. Delayed diagnosis is common due to clinical heterogeneity. The development of clinical trials in A-T is limited by a lack of suitable biomarkers. Abnormal hypointensity of the dentate nuclei (DN) has been demonstrated in some genetic ataxias. We assess the degree of hypointensity of the DN in paediatric and adult A-T patients using the Gupta visual ranking score and report a novel observation of a lack of hypointensity of the DN in adults. Adult A-T patients had significantly lower visual rating scores than paediatric patients (0.07 versus 0.55 respectively). Our findings demonstrate that the normal process leading to hypointensity of the DN is initiated in children with A-T, but the physiological DN hypointensity is lost by adulthood.