Although an isolated clinical case report was published in 1926 and another in 1941, ataxia-telangiectasia (A-T) was not established as a distinct entity until 1957, when it was first delineated clinicopathologically. Susceptibility to sinopulmonary infection was identified as the main cause of death and as the third major component of the syndrome; its heredofamilial nature was documented, and it was designated "ataxia-telangiectasia." In a later review of 101 published cases, lymphoreticular malignancy emerged as the second most frequent cause of death. Although the thymus was found to be absent in the first reported autopsy in 1957 and the serum IgA deficiency was first recorded in 1961, A-T was not established as an immunodeficiency disease until 1963. Thymic abnormality and dysgammaglobulinemia explain the 2 main causes of death, sinopulmonary and neoplastic, but the immunodeficiency is probably not the central defect. It does not appear to explain either of the 2 main clinical diagnostic keys, the ataxia and the telangiectasia, or any of the other seemingly unrealted multisystemic facets of this complex disorder. Some of our most provocative long-term clinical observations and recent pathologic findings in our series of 9 autopsies are discussed.