One hundred patients with a 3rd ventricle width of 12 mm or more were examined for the fifth time in 1976 after an average observation period of 20.8 years. On first admission, a predominant aetiological factor was found in 27 cases. Two patients had air‐encephalogrophical findings indicating normal pressure hydrocephalus. One of these had a shunt operation, however, without improvement.Seventy‐one patients had died, 7 patients were in need of care and supervision, 11 patients were unable to work, the remaining 11 were able to work to some extent. the group studied had a significantly increased mortality rate. the causes of death were divided into three groups: 1. Probably related to the underlying brain disorder; 2. Related to those in an average Norwegian population; and 3. Minor disorder usually not leading to death.The following factors indicated a poor long‐term prognosis: 1. Serious associated disease; 2. A relatively high age; 3. Associated cardiovascular disease; 4. Marked degree of ventricular enlargement; 5. Marked enlargement of the temporal horns; 6. Many and/or marked neurological signs; and 7. Progressive intellectual deterior