The more subtle clinical findings that facilitate early diagnosis and the most provocative long-term clinical observations in our series of patients are emphasized. The most striking pathological findings in our own series of 11 complete autopsies are reviewed in relation to new findings from 57 autopsy reports in the recent literature. Clinical and pathological findings in our oldest patient, who died at age 32, are systematically compared with those of her sister, who died 20 years earlier at age 10 1/2 and who was the subject of the first autopsy in AT, thus providing a rare comparison of the early and late stages of the disease. The clinical and pathological findings, including the gliovascular malformations in the CNS described recently in autopsies on older patients, reveal that AT is characterized throughout its course by multisystemic progeric changes. It is proposed, therefore, that AT can serve as a model for the study of premature aging. Clinical diagnosis, laboratory markers, and special diagnostic procedures, along with general management, immunotherapy, and rehabilitative measures, are reviewed in Part II.