The concentrations of serum amyloid A (SAA) protein, albumin and prealbumin were measured in 22 patients with Alzheimer's disease (AD), in 21 demented patients with Down's syndrome (DS), and in age-matched control subjects for both groups in a 2-year follow-up study. The concentration of SAA was initially elevated in 9 of 22 (41%) patients with AD and in 8 of 21 (38%) patients with DS. After 2-years, 10 of 12 (83%) AD patients and 11 of 14 (79%) DS patients had elevated SAA levels but with fluctuation of the values. None of the controls revealed increase in the SAA level. The concentration of SAA did not correlate with the duration of AD or AD-like process although the highest values were found in cachectic AD patients confined to bed. Although neoplasm or infection were not diagnosed, the presence of occult neoplasm or subclinical infection as a cause of SAA elevation, especially in the DS group susceptible to infection, could not be excluded. Moreover, the fluctuation of SAA values in DS seems likely to be associated with an infection. Albumin and prealbumin levels were decreased in both AD and DS; the prealbumin levels in AD was lower than that in DS.