Alpha 1-antichymotrypsin as a possible biochemical marker for Alzheimer-type dementia. We measured serum alpha 1-antichymotrypsin levels in 38 patients with Alzheimer-type dementia, 89 control subjects, 2 subjects with Down's syndrome, 20 with vascular dementia, 18 with Parkinson's disease, 14 with spinocerebellar degeneration, 15 with cerebrovascular disease without dementia, and 14 with Duchenne muscular dystrophy. Cerebrospinal fluid (CSF) levels of alpha 1-antichymotrypsin were also measured in 15 patients with Alzheimer-type dementia, 26 control subjects, 6 with vascular dementia, 7 with cerebrovascular disorder, and 11 with degenerative disorders. In control subjects, there were no age-related changes or sex differences. Serum and CSF levels were significantly and specifically higher in patients with Alzheimer-type dementia than in other subjects (serum, p less than 0.001; CSF, p less than 0.05). Serum levels of alpha 1-antichymotrypsin were significantly elevated in the early stage of Alzheimer-type dementia, whereas there was no definite correlation between serum levels and the degree of dementia. CSF levels of alpha 1-antichymotrypsin tended to parallel the severity of dementia. Serum levels were not correlated with CSF levels. These data indicate that serum and CSF levels of alpha 1-antichymotrypsin might be independently upregulated in Alzheimer-type dementia. We concluded that the measurement of serum levels of alpha 1-antichymotrypsin could be useful as a screening marker for Alzheimer-type dementia. In addition, CSF levels also could be a useful marker for Alzheimer-type dementia, because they might reflect the state of dementia. 