Bertolotti syndrome is a form of low-back pain associated with lumbosacral transitional vertebrae. We report a case of a 34-year-old woman with breast cancer presented with severe low-back pain. 18F-NaF PET/CT demonstrates increased radiotracer uptake at the transverse-sacral articulation corresponding to hemisacralized left transverse process of L5 vertebra. Increased tracer uptake in patients with lumbosacral transitional vertebrae seems to correlate with lower-back pain symptoms. 18F-NaF PET/CT can be useful in evaluation of some uncommon causes of low-back pain. Bertolotti syndrome should always be included in the differential diagnosis of lower-back pain in young patients.