In a randomized blind study of 69 mental hospital patients over 50 years of age, a highly significant correlation was observed between the clinical symptoms of organic brain disease and the quantity of senile plaques found in the brains at autopsy. A stepwise multiple regression analysis indicated that independent significant predictors of the quantity of plaques were the level or disorientation and the age of the patient. Additional clinical tests for intellectual deterioration, affect lability, and impairment of memory and judgment did not improve the predictive ability. The correlation between incidental pathologic changes unrelated to the senile form of cerebral degeneration and the clinical symptoms was highly significant, though not obvious. Only after removal of the effect of the quantity of plaques on the clinical symptoms (multiple regression analysis) did the effect of other pathologic processes become evident. A significant one-way fixed-effect relationship was noted between increasing quantity of symptoms and increasing quantity of plaques in 48 cases of senile dementia correctly diagnosed by the psychiatry staff. In the control group of 21 patients without senile dementia, the quantity of plaques was correlated with the age of the patients but not with their clinical symptoms.