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Undifferentiated spondylarthropathy is one of the common disease subsets in the group of so-called seronegative spondarthritides. It is not exactly known how often it differentiates into ankylosing spondylitis or other well-defined disease subsets over time. The present study was designed to find out the long-term outcome in this subset. Thirty-five patients diagnosed with undifferentiated spondylarthropathy between January 1987 and December 1988 were recruited. Twenty-two (63%) of them were available for detailed assessment 11 years after the original diagnosis. Their baseline characteristics did not differ from those of the original cohort of 35 patients and were as follows: male:female ratio 19:3, median age of onset 17 years (range 8–39), and median duration of disease 8 months (range 4–24). Clinical features were enthesitis (45%) and inflammatory pain in the back (100%), buttock (77%), hip (64 …