During 1973-75, a total of 446 patients with recent arthritis were examined. The diagnosis in 32 cases was non-classified monoarthritis. After the 3-9 years' follow-up of these 32 patients, two seropositive, definite rheumatoid arthritis cases (6%), and one ankylosing spondylitis case were noted. In the remaining patients the diagnosis was still non-specific arthritis. Of the 31 patients tested, 39% were HLA-B27-positive. The difference between this and the HLA-B27-positivity in the normal Finnish population (14%) is statistically significant (p less than 0.005). At the end of the follow-up the good prognosis of monoarthritis was confirmed: only one erosion developed in one patient's hands or feet.