We report two cases of siblings presenting coexisting non-radiographic axial spondyloartrhritis and joint hypermobility syndrome, complaining of back pain with morning stiffness, enthesitis, peripheral arthralgia, high erythrocyte sedimentation rate and C-reactive protein level and positive HLA-B27. The association of these two conditions is rare, but especially interesting in view of their contrasting features, one causing axial skeleton stiffness, the other a wider range of peripheral joint movements. Coexistence of these two opposite disorders causes confusion in diagnosis and management, resulting in lower quality of life for patients, as they are in pain from the early stages. Therefore, this association is suspected in young individuals with back pain and physical exam findings of peripheral joint hypermobility and axial skeleton loss of mobility.