Ataxia-telangiectasia or Madame Louis-Bar's disease is characterized by progressive cerebelar ataxia, commonly in the infancy, cutaneous and ocular telangiectasies, choreoatethosic movements, tendency for respiratory infections and dementia. There is also a high incidense of tumours of the reticuloendothelial system. The disease is commonly of familiar nature and inherited by recessive autosomic way. Biochemical picture is that of a low or even complete abscence of the "A" fraction of the gamaglobulin as well as alteration in the late reaction of hipersensitivity. The clinical, biochemical and radiological study of a three years old child with the disease since 18 months without others cases in the family is reported.