As in other diseases of undetermined etiology, the diagnosis of ankylosing spondylitis (AS) and related spondyloarthropathies (SpA) is based on clinical and roentgenographic features. The current criteria for diagnosis of some of these diseases are too restricted, and do not recognize the existence of a much wider disease spectrum. For example, radiographically detected sacroiliitis is extremely frequent in AS, but may not be an obligate manifestation, especially in early or atypical forms of the disease. Arthritis involving the axial skeleton, including the sacroiliac joints, can be present in some patients without evidence of erosive disease roentgenographically. The disease spectrum of Reiter's syndrome has also been broadened considerably, and "incomplete" forms of Reiter's syndrome are observed much more commonly than the classical triad of arthritis, conjunctivitis, and urethritis. The term "B27-associated reactive arthritis" has been used in recent years to refer to SpA following enteric or urogenital infections, and the disease spectrum includes the clinical picture of typical Reiter's syndrome. The clinical spectrum of psoriatic SpA has been better clarified. Some of the less well defined B27-associated clinical syndromes include seronegative oligoarthritis, polyarthritis, or dactylitis ("sausagelike" toes) of the lower extremities, and heel pain caused by calcaneal (and tarsal) periostitis. These and other undifferentiated SpA have been ignored in previous epidemiological studies because of the inadequacy of the existing classification criteria. The European Spondylarthropathy Study Group (ESSG) has completed a study aimed at developing preliminary classification criteria for the whole group of SpA patients, with the specific intention of encompassing patients with undifferentiated SpA.