An analysis of the physical and mental symptoms and laboratory findings in 14 cases of fatal acute manic-depressive psychosis has been presented. Essential features were: dehydration and hypochloræmia. Autopsy revealed petechial hæmorrhages into the vital areas of the brain. These hæmorrhages are considered to be secondary to the hypochloræmia, and are the immediate cause of death. Treatment is specific and consists of combating the dehydration and raising the blood chloride level to normal. Nine consecutive cases have been treated with the method described and not a single fatality resulted.The need for research into the cause for the high mortality in cases of acute manic-depressive psychosis, which is a potentially recoverable condition, has been very great. It is hoped that this work will be corroberated by other investigators, and that it may pave the way to materially lower the mortality in these cases.