A rapid screening method is described for detecting amyloid angiopathy (AA) in the meningeal vessels with the use of thioflavin-S and fluorescence microscopy. AA was found most frequently in the brains of patients in whom senile degeneration of the Alzheimer type was complicated by severe arteriosclerosis, vascular hyalinosis and multiple infarcts (73 percent). In 69 randomly drawn mental hospital patients over the age of 55, the degree of AA was significantly correlated with the degree of organic symptomatology (r = .417, p less than .001). Multiple regression analysis, however, revealed that AA as a predictor of clinical symptoms was highly dependent upon another predictor, i.e., the quantity of senile plaques in the brain. In contrast, a highly significant and independent correlation was found between the clinical symptoms and AA in a control sample of 21 patients who had mental illnesses other than senile dementia (r = .683, p less than .001). In these patients the plaque count was very low and arteriosclerosis was prevalent. Among the clinical symptoms, Affect Lability was consistently most highly correlated with AA, whereas Disorientation was most highly correlated with the quantity of plaques.