This article aims to review the role of the functional neuroimaging modality of positron emission tomography (PET) in the early diagnosis of Alzheimer's disease (AD). Clinical diagnosis in the early disease stages is difficult and treatments are emerging which rather than reversing structural damage are likely to slow or halt the disease process. While currently no routine diagnostic test confirms AD presence, imaging techniques are an important expanding field in biological neuropsychiatry. The challenge for neuroimaging methods is to achieve high specificity and sensitivity in early disease stages. Glucose metabolic PET imaging with fluorodeoxyglucose (FDG) has the potential to detect very early neocortical dysfunction before even abnormal neuropsychological testing is obtainable. The implications are for the identification of minimally symptomatic patients that could benefit most from treatment strategies, as well as the monitoring of treatment response and possible therapeutic deceleration of the disease. FDG PET correlates with AD neuropathology and is able to indicate disease progression or severity, meeting both functional neuroimaging prerequisites in diagnosing AD. A combination of functional neuroimaging with different techniques should be able to provide highest diagnostic specificity in diagnosing dementia. This may even lead to a new classification of dementias according to differences in the causative aetiology.