This paper reviews the concept and outcome of undifferentiated spondyloarthritis (SpA) as reported in retrospective and prospective studies from different parts of the world. Although the designs and definitions vary across different studies, clearly a significant proportion of patients with undifferentiated SpA will fulfill ankylosing spondylitis within 10 years in association with certain prognostic factors. The paper also examines the concept of axial SpA and its importance as an early indicator of ankylosing spondylitis. Ultimately, the recognition of undifferentiated SpA and axial SpA may lead to early treatment with highly efficacious drugs.