The levels of α1-antichymotrypsin (ACT) in cerebrospinal fluid (CSF) from 66 sporadic Alzheimer's disease (AD) patients and 54 normal controls were measured by enzyme immunoassay and compared. There was no correlation (r=0.259, n=54) between the ACT level and normal aging. The levels of ACT were significantly higher in the total AD group (p<0.01) than in the normal control group. Dividing AD patients into early onset AD (n=27) and late onset AD groups (n=39), the mean level of CSF ACT in the late onset AD group was significantly higher than that in the normal control group (p<0.001) and that in the early onset AD group (p<0.01). Thus, the level of ACT in CSF is closely associated with late onset AD.(Internal Medicine 34: 481-484, 1995)