Ataxia telangiectasia is a multisystem disease with an autosomal recessive inheritance. It is characterized by progressive cerebellar ataxia, oculocutaneous telangiectasia, humoral and cellular immunodeficiencies and high incidence of neoplasia and radiosensitivity. A 5 year retrospective survey included 24 patients belonging to 17 families. Cerebellar ataxia was the first clinical symptom and was usually noticed when the child began to walk. Mean age of onset was 2.9+/-1.8 years. Oculocutaneous telangiectasia was present in 17 cases and appeared between 2 and 8 years and then spread in a characteristic symmetrical pattern. When ocular telangiectasia was absent (6 cases), the diagnostic of ataxia telangiectasia was retained on oculomotor apraxia (2 cases), recurrent sinopulmonary infections (3 cases) and/or a sib with typical ataxia telangiectasia (1 case). Recurrent sinopulmonary infections, absence or low serum level of IgA (78 p.100) and lymphopenia revealed immunodeficiency. Among 12 patients, chromosomal instability was observed in 5. Balanced rearrangements involving chromosomes 2, 7, 14, 22, 1, 3 and 11. The responsible gene, ATM, encodes a large protein kinase with a phosphatidylinositol 3-kinase-like domain. Ataxia telangiectasia patients have a 100 fold higher risk of cancer than the general population. We reported, in the same family two patients who developed neoplasia, (lymphoma and leukemia). During follow-up, a progressive worsening was observed in all cases. Three patients have died.