AbstractINTRODUCTIONThe cognitive and neuroimaging evolution during dementia with Lewy bodies (DLB) from the prodromal phase (Pro‐DLB; subjective cognitive impairment [SCI] to mild cognitive impairment [MCI]) according to amyloid beta (Aβ) status is poorly understood.METHODSThe decline of Lewy‐Memento patients with SCI or MCI was compared according to Aβ status across four groups: Pro‐DLB, prodromal Alzheimer's disease (Pro‐AD), Pro‐DLB+AD, and a group without prodromal DLB and AD (no symptoms [NS]). We observed the evolution of cognitive, functional, quality of life measures, brain volumetry, and metabolism on fluorodeoxyglucose positron emission tomography.RESULTSIn the Pro‐DLB and Pro‐DLB+AD groups, Aβ+ patients had more cognitive and functional decline than the Aβ– patients. In the Pro‐AD and NS groups, Aβ+ patients had more functional decline. Aβ+ Pro‐AD showed a greater volume decline of the brain (left insula).DISCUSSIONThe presence of amyloid lesions worsens very prodromal DLB patients over time, both cognitively and functionally, but without increasing atrophy.Highlights Patients at a very prodromal stage, subjective cognitive impairment or mild cognitive impairment, had a clinical diagnosis of either prodromal Alzheimer's disease (Pro‐AD), prodromal dementia with Lewy bodies (Pro‐DLB), Pro‐DLB+AD, or no diagnosis. Amyloid beta positive (Aβ+) patients had more functional decline, whatever the group. Aβ+ DLB patients (Pro‐DLB and Pro‐DLB+AD) had more global cognitive (Mini‐Mental State Examination) decline. Aβ+ Pro‐AD patients showed a greater volume decline of the left insula.