Ataxia telangiectasia (AT) is a chromosomal instability syndrome with autosomal recessive inheritance, it is caused by more than 500 mutations of the ATM gene, which is involved in the cellular response to DNA damage. The diagnosis becomes difficult due to the evolution of the disease, their poor knowledge, and limited access to diagnostic tests. Chromosomal damage induced by ionizing radiation (IR) assay is still a sensitive method for early diagnosis, and it is essential for better management and genetic counseling. This paper shows diagnosis and follow-up in four cases with AT.