We sought to identify a marker for Alzheimer's disease (AD) for antemortem diagnosis. To determine whether the detection of reduced blood flow in the parietotemporal cortex, shown by single photon emission CT (SPECT), and of medial temporal lobe atrophy, shown by magnetic resonance imaging (MRI), would be useful in diagnosis, we studied 38 patients with AD diagnosed by the NINCDS-ADRDA criteria and 26 healthy elderly controls. Parietotemporal hypoperfusion was qualitatively assessed by physicians who were unaware of the clinical diagnosis, and the severity of medial temporal lobe atrophy was quantitated by planimetric and linear measurements. Although an accurate diagnosis of AD was made in 80% or more of the patients by SPECT or MRI studies alone, the combination of SPECT and MRI gave a higher diagnostic accuracy, with a sensitivity of 95% and a specificity of 92%. Since regional functional or structural changes were detected in 92% of early or mild patients, including possible AD, the combination of SPECT and MRI studies were useful in the early diagnosis of AD. Findings suggest that a functional abnormality in the parietotemporal lobe and an atrophic change in the medial temporal lobe are characteristic of AD, and that SPECT and MIR regional changes may be useful as antemortem diagnostic markers.