Prealbumin (transthyretin) quantified in CSF was negatively correlated with degree of dementia in 24 patients with dementia of Alzheimer type (DAT). There was neither a significant correlation between prealbumin in serum and degree of dementia nor between prealbumin in CSF and number of T lymphocytes in serum. There was no such relationship in 7 patients with multi-infarct dementia. Prealbumin was also quantified in 4 non-demented control groups, one consisting of 14 age- and sex-matched neurological patients, 17 with MS, 6 with ALS and 10 patients who had had a subarachnoid hemorrhage. Prealbumin was significantly lower in the last group. The study suggests that there may be a relationship between prealbumin levels in cerebrospinal fluid and degree of dementia. It is suggested that this may be caused by affection of the choroid plexus.