Ataxia telangiectasia (A-T) is a rare autosomal recessive multisystem disease. The diagnosis of A-T is based on the typical clinical picture: ataxia and telangiectasia. However, an increase in (alpha-fetoprotein (AFP) level and the identification of the A-T mutated gene (ATM) assist in an early diagnosis. Here we report two cases of A-T diagnosed in our hospital (case 1: a 7-year-old boy; case 2: an 8-year-old girl). Both of these patients had typical clinical pictures of ataxia and telangiectasia, AFP was also increased (case 1:471.2 ng/dL; case 2: 196 ng/dL). T-cell dysfunction was noted in both patients. Case 1 had IgG2 deficiency and case 2 had IgA, IgG2 and IgG3 deficiency. Case 2 developed malignant lymphoma at 9 years of age and died of pneumonia with respiratory failure at 10 years of age. Because of rhe rarity of A-T in Taiwan, we report two cases to help pediatricians make an early diagnosis of A-T if they have a patient with progressive ataxia and oculocutaneous telangiectasia.