The life span of individuals with DS has gradually increased since the 1920s. The DS individual now has an average life expectancy of 35 years. Despite advances in the health care of the retarded and improvements in the quality of institutional care, the overall mortality rate remains elevated by five-fold. Specific mortality rates from respiratory diseases (particularly pneumonia), infectious diseases, congenital heart disease, leukaemia and neurological disorders are still substantially increased. Disorders of immunological functioning, particularly T-cell mediated, appear related to this increased vulnerability, although further research is necessary. The periods of highest risk are during infancy, when congenital heart disease, leukaemia and respiratory diseases are most lethal, and late adulthood, when Alzheimer-type dementia and declining immunological function appear to be significant factors.