The use of radioactive yttrium (90Y) in the treatment of rheumatoid knee joints is described. Clinical results in 48 knees after a mean observation period of more than 3 years are reported: 60 percent excellent or good at one year, 46 percent at two years, 33 percent at three years and 17 percent at four years. The number of failures or relapses exceeded 50 percent between 30 and 36 months. The average rate of relapse in successful cases was 12.8 percent per year. The most important factors associated with unsatisfactory result were advanced articular destruction and progression of the systemic disease.