The C9ORF72 hexanucleotide expansion is a major pathological expansion pattern found in patients with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (C9FTD/ALS). We describe a patient in whom early clinical evaluation, MRI and fluorodeoxyglucose (FDG) positron emission tomography (PET) findings failed to definitively differentiate between FTD and Alzheimer's disease (AD), whereas (11)C-Pittsburgh compound B (PiB) PET was negative for amyloid pathology. He later developed ALS symptoms, and post mortem neuropathological findings were diagnostic of FTD-ALS, while no findings suggested AD. His sister was diagnosed with FTD, and the C9ORF72 expansion was detected in both siblings. We conclude that ¹¹C-PiB PET imaging may help the early differential diagnosis between AD and FTD, including C9FTD/ALS.