INTRODUCTION: Lumbosacral transitional vertebra is an anatomical variation of the fifth lumbar vertebra in which an enlarged transverse process can form a joint or fusion with the sacrum or ilium. The association of that variant with low back pain and the change in the biomechanical properties of the lumbar spine is called Bertolotti's syndrome.CASE PRESENTATION: We report a case of a 40-year-old male patient with chronic low back pain extending to the left buttock, just above the ipsilateral sacroiliac joint. Radiographic investigation revealed an anomalous enlargement of the left transverse process of the fifth lumbar vertebra forming a pseudarthrosis with the infrajacent ala of the sacrum.CONCLUSION: In young patients with back pain the possibility of Bertolotti's syndrome should always be taken in account.