Eighty-eight patients with possible ankylosing spondylitis (AS) were selected for this followup study. They showed normal or at most suspicious radiographic findings of the sacroiliac joints. After 5 years' followup, 24, and after 10 years 32 patients (59% of the 54 finally available, 36% of the 88 original patients) had definite AS. In 12 individuals, AS could be excluded. Of the 10 remaining patients, 6 still had possible, and 4 had undifferentiated spondyloarthropathy. A comparison between HLA-B27 positive and negative patients showed a significantly increased frequency of definite AS or possible and undifferentiated spondyloarthropathy (p less than 0.05) in the group of HLA-B27 positive patients. The development of AS was characterized by a prolonged course: radiological sacroiliitis became evident after at least 9 +/- 6 years, radiological signs of spinal involvement after 11 +/- 6 years mean disease duration. After 18 +/- 6 years 25 (78%) of 32 patients with AS still maintained good or sufficient functional capacity, indicating a good functional prognosis in the great majority of the patients.