To evaluate the clinical significance of serum alpha 1-Antichymotrypsin (ACT) as an early diagnostic marker of senile dementia of Alzheimer type (SDAT), we measured 333 healthy and not demented elderly subjects, 27 cases SDAT and 25 cases of vascular dementia (VD). For the measurement, a new high-sensitivity method, double antibody radioimmunoassay method was developed. In healthy elderly subjects, the mean value of serum ACT was 0.229 mg/ml. A tendency towards increase of ACT with aging was noted but was not significant. The serum level of ACT in the SDAT patients was significantly higher (0.309 mg/ml) compared with the healthy elderly subjects and the VD patients (0.226 mg/ml) (p < 0.01). We concluded that in the patients with SDAT, there was an overproduction of ACT and the serum value of ACT was markedly elevated. The measurement of serum ACT is very useful (sensitivity = 88.9%, specificity = 68.7%; cut-off value = 0.250 mg/ml) for the early differential diagnosis of senile dementia.