To investigate the relationship between mental depression and death among elderly persons, a prospective study was conducted. 2, 166 inhabitants aged from 60 to 74 years in the Hidaka district of Hokkaido Prefecture, Japan were employed. From December 1990 to January 1991 we estimated the degree of depression of the subjects, using the self-rating depression scale (SDS) developed by Zung. The subjects were classified into 3 groups according to the average SDS score ( Group A, -, 2.40), and were followed up periodically until February 1995 to confirm whether they were still alive. The highest mortality was noted in Group C which consisted of those who had severe depression. Their mortalities for cancer, pneumonia and suicide were significantly high. After the adjustment of age, number of current diseases, smoking state and sex, a similar tendency was noted. Mental depression increased mortality of the aged. Part of this mechanism might be explained by the suppression of immunological resistance