The term undifferentiated spondyloarthropathy (uSpA) refers to patients with clinical and roentgenographic features suggestive of spondyloarthropathies but not fulfilling the diagnostic or classification criteria for any of the currently established disease categories. The frequency and clinical spectrum of uSpA have been ignored in previous epidemiologic and clinical studies. A generally accepted nosologic concept and definition of uSpA may be needed to overcome this issue. So far the recently developed ESSG criteria have the broadest basis of consent, at least for several European centers. With the use of the ESSG classification criteria the real prevalence may be better defined in the future and the early classification of such patients in clinical practice should be advanced. Nevertheless, the diagnosis of uSpA is only a working label with the implicit demand to solve the clinical conundra by follow-up or even better by identifying the causative or triggering infectious agents.