Atlantoaxial subluxation in undifferentiated spondyloarthropathy is extremely rare and has not been reported. We describe a case of 27-year-old man who was diagnosed as undifferentiated spondyloarthropathy with atlantoaxial subluxation with an initial complaint of painful swelling of right 1st metatarsophalangeal joint and posterior neck pain. Roentgenograms showed sclerotic change and increased hazziness in right 1st metatarsophalangeal joint. Magnetic resonance images and roentgenograms of the cervical spine showed inflammation of odontoid process and atlantoaxial subluxation. Bone scan showed hot uptakes in left sacroiliac joint, right 1st & 4th metacarpophalangeal joints and 1st metatarsophalangeal joint. HLA-B27 gene was positive. Spontaneous atlantoaxial subluxation and undifferentiated spondyloarthropathy was diagnosed and conservatively treated with oral medication. Currently, there is no definite neurological sign. Early recognition and awareness of potential clinical complications is important in preventing compressive damage on central nervous system. (J Korean Acad Rehab Med 2008; 32: 361-365)