Survival and causes of death of 218 patients with Alzheimer's disease (AD) and of 115 patients with multi-infarct dementia (MID) were examined. The patients were originally found in a community-based epidemiological survey of dementia, and all patients with AD or MID alive on the prevalence day were included. The 6-years survival rate for AD was 21.1% vs. the expected rate 48.5%, that for MID 11.9% vs. 45.2% expected. A comparison of relative survival rates suggested that MID carries a less favorable survival prognosis than AD. The mean durations were: AD 5.7 years and MID 5.2 years; median duration being 5 years in both diseases. The excess mortality in both AD and MID was independent of age. In AD, the survival rate decreased with increasing severity of dementia, while in MID the mortality was the same regardless of the severity of the dementia. The dementia disorder was the underlying cause of death in 68% of AD patients, and in 38% of MID patients, bronchopneumonia being the most frequent immediate cause of death in both groups. As a cause of death, acute cerebrovascular accidents occurred more often in MID patients than in the general population of comparable age. Malignant diseases were less frequent as a cause of death in both dementia groups than in the general population.