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%), shoulder (18%), knee (82%), ankle (77%), and hand and wrists (50%). There was no restriction in spinal movement. Family history was positive in two cases. Radiologically, the only finding was grade I sacroiliitis in 17 patients (77%). Human leukocyte antigen (HLA)-B27 was positive in all. Functionally, all were in class I. During follow-up, one patient developed psoriatic skin lesions after 9 years. Uveitis developed in four patients (18%). After a median follow-up of 11 years, 15 (68%) had ankylosing spondylitis, one developed psoriatic arthritis, four remained undifferentiated, and two had natural remission. Functionally, 19 patients (86%) were in class I and three (14%) were in class III. No patient had bamboo spine, but three underwent total hip replacement. Thus, a majority of patients (68%) with undifferentiated spondylarthropathy gradually developed ankylosing spondylitis of mild severity.