A double-blind study of erbium 169 injection into rheumatoid digital joints was carried out with saline as control. 201 joints in 36 patients were studied (137 metacarpophalalangeal, 64 proximal interphalangeal). Erbium 169 was injected into 121 joints and saline water into 80 joints. Local injection of corticosteroids was given to both groups. A definite improvement was observed in 55% to 58% of cases with erbium 169 (+prednisolone acetate) and in 26% to 28% of cases with saline (+prednisolone acetate). The difference was highly significant.