Plasma cortisol levels and other factors including thyroid hormone in patients with Alzheimer's type (n = 10), vascular type (n = 10) or mixed type (n = 10) senile dementia were compared with those in non-demented senile controls (n = 10). Plasma cortisol levels at 8:00 a.m. in Alzheimer's type dementia and mixed type dementia were 17.3 +/- 4.3 micrograms/dl (mean +/- SD) and 15.6 +/- 2.3 micrograms/dl, respectively. These values were significantly higher (p less than 0.005 and p less than 0.01) than those found in the control subjects (12.0 +/- 3.1 micrograms/dl). Plasma cortisol levels in vascular-type dementia (14.4 +/- 6.3 micrograms/dl) did not differ significantly from those in the controls. Plasma ACTH in senile dementia of the Alzheimer's type was lower, but not significant as compared with that in normal controls. In three subgroups of senile dementia and normal controls, plasma cortisol levels inversely correlated significantly with the degree of cognitive function. Plasma levels in TSH-thyroid system and blood pressure did not show any significant change in three types of senile dementia. These data suggest that senile dementia of the Alzheimer's type accompanies relatively and primarily high plasma cortisol levels and this may associate with cognitive dysfunction in Alzheimer's type senile dementia.