Direct analysis of amyloid proteins in human plasma will promote rapid screening of brain amyloidosis, the earliest pathological signature of Alzheimer's disease. We developed a microflow liquid chromatography-targeted mass spectrometry assay for quantitation of four intact β-amyloid proteins starting from 1 mL of human plasma samples. This method showed 90% accuracy for predicting brain amyloid using plasma Aβ42/Aβ40 values from 36 cognitively normal individuals in a prospective clinical study (raw data deposited in MassIVE, Data set ID MSV000087451). Our method may contribute to early diagnosis of Alzheimer's disease.