T lymphocyte development occurs primarily during fetal life through cell-to-cell interactions with epithelial and dendritic cells. Evidence now supports the concept that thymic function continues during adulthood, which may explain why thymic neoplasms are associated with immunodeficiency and autoimmune illness. A broad variety of autoimmune conditions are observed in patients with thymic tumors, especially hematological and paraneurological syndromes, including aplastic anemia, pure red cell aplasia and myasthenia gravies, encephalomyelitis, cerebellar degeneration, sensory neuropathies, and Lambert-Eaton myasthenic syndrome. A more thorough understanding of the immunological dysfunction in patients with thymoma and thymic carcinoma promises to contribute to the overall understanding of human immunology and improve the clinical management of patients.