The value of functional and morphological neuroimaging in the early diagnosis of Alzheimer's disease (AD) is still debated. Described here are cerebral perfusion and linear measures of medial temporal lobe atrophy in 2 monozygotic twins discordant for AD who were investigated with technetium 99m-hexamethylpropy-leneamineoxime single-photon emission tomography (SPET) and magnetic resonance (MR) imaging. Both showed pathological cortical perfusion findings on SPET, while medical temporal lobe atrophy was present only in the affected twin. MR measures of medial temporal lobe atrophy have greater agreement with clinical data than do SPET measures of cerebral perfusion. Evaluation of atrophy may be useful in the early diagnosis of AD.