The objective of the study was to illustrate the utility of positron emission tomography (PET) imaging using [C]PIB and [F]FDDNP together with cerebrospinal fluid (CSF) measures of amyloid-beta1 to 42 (Abeta42), total tau (t-tau) and tau phosphorylated at threonine 181 (p-tau) in the in vivo diagnosis of specific dementia syndromes. Two siblings fulfilling diagnostic criteria for familial Alzheimer's disease (AD) were investigated using [C]PIB and [F]FDDNP PET in combination with CSF measures of Abeta42, t-tau and p-tau. PET data were compared with paired [C]PIB and [F]FDDNP data from age-matched sporadic AD patients (n=9) and healthy controls (n=6). [C]PIB retention and CSF levels of Abeta42 in both patients resembled those of controls suggesting the presence of nonamyloid pathology. Genetic testing confirmed the absence of mutations in the presenilin 1 gene in 1 patient; subsequent testing revealed the R406W tau mutation in both individuals leading to a diagnosis of frontotemporal dementia [F]FDDNP retention broadly correlated with CSF levels of t-tau and p-tau. Despite both individuals harbouring the same mutation, [F]FDDNP retention and CSF t-tau and p-tau were elevated in 1 patient, but not in the other. [C]PIB imaging and CSF measures of Abeta42 are useful in refuting the presence of underlying amyloid pathology. This, in combination with elevated levels of CSF t-tau and p-tau, has potential value in differential diagnosis of frontotemporal dementia from AD.