This report addresses the clinical differentiation of Alzheimer's disease (AD) from frontotemporal dementia (FTD), including Pick's disease. The accuracy of a clinical diagnosis of a dementing disorder is determined in part by the prior probability (base rates) of the disorder, which predicts an overwhelming likelihood of a diagnosis of AD, because the prevalence of AD is much greater than FTD. The clinical features of the disorder also determine the accuracy of diagnosis. Recent studies have reported an improvement in the differential diagnosis of FTD, utilizing the Lund-Manchester criteria. Patients with FTD typically have early noncognitive behavioral changes with relatively spared cognition, frontal atrophy and enlargement of the Sylvian fissures on CT and MRI scans, and frontal-temporal deficits on SPECT or PET scans. In contrast, AD patients have early cognitive changes with relatively preserved personality and behavior, hippocampal and medial-temporal lobe atrophy on CT or MRI scans, and parietotemporal SPECT or PET deficits.