Amyloid positron emission tomography (PET) has emerged as an in vivo technique for visualizing the accumulation of fibrillar amyloid-beta (Aβ). Carbon-11 Pittsburgh compound-B (C-11 PIB) has gained widespread acceptance as a standard amyloid PET probe. We report a case of a 53-year-old woman with probable Alzheimer's disease (AD) confirmed by neuropsychological assessment and cerebrospinal fluid analyses. Fluorine-18 fluorodeoxyglucose PET depicted an AD-typical pattern of cortical hypometabolism. Unexpectedly, C-11 PIB PET detected no cortical Aβ depositions. The present case challenges the notion that a negative C-11 PIB scan excludes AD and argues for a comprehensive diagnostic workup.