In an 8-year follow-up of a prospective study of recent arthritis, 103 patients were found to have erosive and 97% seropositive rheumatoid arthritis. A 20% incidence of hip joint involvement was observed among them. Moderate radiological changes could be seen in 10%, severe destruction (Larsen 4 or 5) in 3%, and acetabular protrusion in 1% of the patients. It was estimated that the minimum annual need of total hip replacement in rheumatoid arthritis in Finland would be 470. This is about 100 per million in the overall population. At the end of the follow-up the Larsen X-ray index was highly significantly (p less than 0.001), ESR significantly (p less than 0.005), and CRP clearly (p less than 0.03) higher in the 13 patients with radiologically confirmed hip changes than in the 90 without them. Thus their hip destruction is attributable to a more severe disease condition and not to the glucocorticoids administered. Furthermore, HLA-B27 was positive in 69% of the above-mentioned 13 patients compared with 27% of the rest, and this difference (p less than 0.005) may also explain the poor prognosis for true rheumatoid arthritis patients.