The need to differentiate between dementia and the so-called functional psychiatric diseases of old age is of therapeutic as well as social importance. The main symptoms of dementia--lack of memory, disorientation and cognitive disturbances--are much less clearly defined than would be desirable, despite the neuropathological changes regarded as underlying senile dementia. The disease does not always have a progressive course. Confusion is also possible with other forms of dementia, while the symptoms of dementia can also be found in so-called functional psychiatric diseases. On the basis of the literature and/of our own research, it appears that a sharper definition of senile dementia is possible. Besides clinical criteria one can make use of--partly recently introduced--diagnostic techniques, specifically a structured psychiatric interview for old people, a neuropsychological testbattery, EEG, visual evoked response techniques and CT-brain scanning. The value of these diagnostic techniques for the diagnosis and the assessment of the prognosis of senile dementia can only be determined in a follow-up study. The set-up of such a follow-up study is described.