Thirty-eight demented patients were clinically investigated and classified into a multi-infarct dementia (MID) and senile dementia (SD) group. Total protein, albumin, immunoglobulin G, agar gel electrophoretic protein and iso-electric focusing pattern were determined in CSF and serum as well as the lactate dehydrogenase, LD-iso-enzyme pattern and uric acid. No significant differences were found regarding total protein, albumin and immunoglobulin G. The agar gel electrophoresis showed a tau-globulin increase in 18% of the SD-group against 6% of the MID group. The patterns were, however, plasma-like in 56% of the MID group against 23% of the SD group. The iso-electric focusing pattern showed an abnormal band in some SD-patients which was not found in the MID group. The uric acid and LD-enzyme concentration was equal in both CSF and serum. An index [Formula: see text] was calculated to give an approximate correction for the influence of different permeability of the blood-brain and CSF barrier. A significantly higher LD-index was found in the SD-group, suggesting a continuous cell degeneration.