The immunological profile was evaluated in 12 adults affected by Down's syndrome. Our findings showed: increase of serum IgG, IgA and Gammaglobulins, decrease of IgM, presence of auto-antibodies and increased antibodies response after antigenic stimulation (typhoid vaccination). Some tests, connected with T lymphocytes functions, were also abnormal: percentage of E-active rosettes, cutaneous sensibilization with DNCB and lymphocyte stimulation index with PHA. Our findings suggest a T lymphocyte deficit, with loss of immunological surveillance and therefore of the control over antibody mediated immunological reactions. The immunological alterations observed in our adult patients with Down's syndrome were more extensive and severe than those found in young subjects. The possible relevance of these findings is discussed as well as the incidence of Alzheimer's dementia in adult patients with Down's syndrome.