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Radiation synovectomy is a useful treatment modality in patients with refractory synovitis. We have developed a 188Re-tin-colloid as a new radiopharmaceutical agent and investigated its efficacy and safety in patients with rheumatoid arthritis. Radiation synovectomy was performed using 188Re-tin-colloid in 22 knees from 21 rheumatoid arthritis patients refractory to intra-articular corticosteroid injection. The efficacy and safety of administration of 370-1110 MBq of 188Re-tin-colloid were evaluated after 1, 3, 6, 9 and 12 months. Pain intensity on a visual analogue scale decreased significantly 12 months after therapy (mean±SD: 68.0±26.1 mm vs. 25.1±23.4 mm; P0.0001 by the paired t-test). Pain decreased in 19 cases (86.3), joint tenderness improved in 14 cases (63.6) and joint swelling was reduced in all cases (100). 188Re-tin-colloid was safe. The residual activity of 188Re in the blood was 0.077±0.25 of the injected dose. The radioactivity of 188Re in the urine was 0.14±0.13 of the injected dose. Transient reactive synovitis was observed in 18 cases (81.8). No clinical side-effects or abnormalities in leucocyte count, platelet count, liver function tests or urine analysis were observed in any patient. In conclusion, in this first study of radiation synovectomy using 188Re-tin-colloid for patients with rheumatoid arthritis, the treatment resulted in the improvement of arthritis and was well tolerated.