In 33 patients with chronic arthritis of the knee, 48 knees were treated with an intra-articular injection of 5 mCi yttrium silicate (90Y). There were 27 patients with rheumatoid arthritis (RA) and 6 with osteoarthrosis (OA); the mean follow-up period was 33 months. At clinical investigation after 1 year, no signs of pain or swelling were found in 15 knees. In most cases, pain and swelling improved subjectively, with a mean duration of 11 months; in 20 knees, the improvement lasted more than 22 months. When radiographs showed severe destruction, 90Y treatment was unsuccessful, but an important new finding was that most patients with mild or moderate radiological abnormalities appeared to have a long-lasting improvement. The result did not correlate with erythrocyte sedimentation rate (ESR), haemoglobin or Rose titre at the time of injection or at follow up, suggesting that the result of the treatment is more dependent on local factors than on the disease activity. The results of 90Y treatment in 6 OA knees with persistent swelling were promising regarding swelling, even in patients with moderate radiological abnormalities. The main side-effect was a sometimes painful swelling of the knee, which was always successfully treated with an intraarticular corticosteroid injection. In 90Y-treated knees, the incidence of unstable joints was not significantly higher than in non-treated knees. In conclusion, 90Y synovectomy may be a successful treatment for patients older than 50 years with chronic arthritis of the knee due to RA and probably also OA, even when moderate radiological abnormalities are present.