The protein melanotransferrin (p97) is associated with the brain lesions of Alzheimer's disease (AD) and is a potential marker of the disorder. We measured serum p97 concentrations in 211 subjects: 71 patients with AD, 56 patients with non-AD-type dementia, and 84 normal control subjects. Serum p97 concentrations were elevated 3- to 4-fold in AD (median 15.00 pg/microl, interquartile range 10.20-17.00 pg/microl) as compared to non AD dementia (2.85 pg/microl, 1.93-7.15 pg/microl) and normal controls (3.20 pg/microl, 2.55-3.95 pg/microl). The mean elevation was significant at 13.54 +/- 3.72 pg/microl, even in the 38 subjects with mild AD (CDR stage 0.5-1). Receiver operating characteristic analyses confirmed an optimal diagnostic threshold of 10.0 pg/microl, which yielded over-all accuracy of 0.882 to 0.915. Serum p97 is a candidate marker of AD, even in the early stage when clinical diagnosis is most uncertain.