The results are reported of the treatment of 33 patients with long-standing, therapy-resistant synovitis and effusion of the knee by intra-articular injection of 90Yttrium. In 25 patients with bilateral gonitis the effect of the treatment was compared with the effect of local corticosteroid therapy to the contralateral knee. The irradiated group showed statistically significantly better therapeutic results at 3, 6, and 12 months and the effect weakened in proportion to the length of the follow-up period. At the final check-up at 3.5 years, 52% of this group showed improvement, as against 26% of the controls. The difference in radiological deterioration at the end of the observation period between the two groups was not significant. Local remission in the treated joint in spite of high general activity was recorded in 7 patients. In 4 patients, in whom the results were unsatisfactory, severe articular destruction and instability were noted prior to therapy. Assessment of the correlation between clinical diagnosis and therapeutic results shows that the treatment seems to be most successful in patients with rheumatoid arthritis and unsatisfactory in patients with psoriatic arthritis. General reactions were observed in 1, and local reactions in 2 patients. Irradiation therapy seems to be the treatment of choice in patients over 40, with long-standing knee arthritis of rheumatic origin.