Forty patients with senile dementia were evaluated prospectively with laboratory and radiologic investigations, including computerized axial transverse tomography. Five patients were believed to have potentially treatable illnesses causing the dementia. After treatment, the dementia in one patient with hypothyroidism was completely resolved. One patient with pernicious anemia was markedly improved, and a second patient with hypothyroidism was somewhat improved. One patient with a brain tumor was not treated, and a severely demented patient with possible normal pressure hydrocephalus died shortly. After shunting without improvement. The discovery of these unsuspected illnesses and the gratifying response to treatment in some suggests the importance of systematic evaluation in all patients with senile dementia.