In the interface between medicine and law, physicians may be asked to comment on a patient's life expectancy. Life expectancy is based on life tables, eg, from the US census, and are published in National Vital Statistics reports. The life expectancy for a particular patient with a serious medical illness or disability cannot be determined from such tables; rather one must investigate the life expectancies associated with the patient's particular disease or condition. Standardized mortality ratios have a complex relationship to life expectancy, depending on the patient's age and specific conditions. For example, excellent data are available regarding life expectancy for individuals with spinal cord injury (SCI) and are sorted in various ways, eg conditional life expectancy calculated for persons who have survived the first year after an SCI. Major depressive disorder is associated with increased mortality due to suicidality and other causes (eg, cardiovascular disease or respiratory disease); men with mental disorders live twenty years less and women live fifteen years less compared with the general population. To testify about life expectancy in a person with neurological or psychiatric disability, the experience of a personal physician is not adequate—familiarity with epidemiological data is a prerequisite. This article is based on a wide and recent body of literature concerning life expectancy and gives an indication of some of the complexities regarding life expectancy.