Of 88 selected patients with possible ankylosing spondylitis (AS) 54 (61%) participated in two phases of a 10 years' follow-up study. Thirty-two (59%) developed definite AS according to the New York criteria, 10 (19%) had possible/undifferentiated seronegative spondylarthropathy (SSA) and 12 patients had other diagnoses. Only 3 (9%) of 35 patients with sacroiliitis did not fulfill the New York criteria for definite AS until the last examination. Sacroiliitis and radiological spinal signs of AS appeared rather late above a mean age of 40 years and after a mean disease duration of more than 10 years. After 18 years mean disease duration 25 (78%) of 32 AS patients had good or sufficient functional capacity indicating an overall good functional prognosis. HLA B27 typing proved to be useful in patients with possible early AS: 29 (71%) of 41 B27 positive and 3 (23%) of 13 B27 negative patients developed definite AS (p less than 0.005). A combination of the B27 test with data of the history, clinical, laboratory, and radiological examination proposed as early diagnostic criteria detected patients with the outcome diagnosis of definite AS with even higher significance (p less than 0.001). These criteria were also useful in the identification of patients with possible or undifferentiated SSA. The recently recognized entity of undifferentiated SSA should only be diagnosed after long term follow-up.