We reviewed the case notes and X-rays of all patients with knee arthritis treated with yttrium-90 for the first time at a single institution from November 1981 to November 1995. Outcomes were assessed as 'improved' or 'not improved' by review of the case notes at 3, 6 and 12 months, and by the absence of further intra-articular (IA) steroid injections. Of the 121 knees treated, 87 had adequate follow-up information to allow an assessment of outcomes. Overall, 46% (95% CI 36-57) were improved at 12 months and 37% (95% CI 27-47) had no further IA injections (mean follow-up of 3.5 yr). Knees with osteoarthritis (OA) fared significantly worse with 10% (95% CI 0-29) vs 51% (95% CI 39-63) improved at 12 months (P < 0.05). Knees younger than 30 appeared to do better with 78% (95% CI 51-100) vs 28% (95% CI 17-45) having no further IA injections (P < 0.02). Knees with normal X-rays (Kellgren grade 0-1) did significantly better than those with more severe radiographic abnormalities (Kellgren grade 3-4), with 56% (95% CI 40-73) vs 24% (95% CI 8-40) improved (P < 0.01). Radiosynovectomy with yttrium-90 for knee arthritis appears to be of less value for patients with OA or with secondary OA changes on X-ray, and may be of more value for younger patients and those with spondyloarthropathies.