Fifteen patients with bilateral, symmetrical, chronic pyrophosphate arthropathy of the knee were given intra-articular injections of yttrium-90 (5 mCi) plus steroid (triamcinolone hexacetonide, 20 mg) into one knee, and saline plus steroid into the other (control) knee. Allocation of the 90Y injection was random and double blind. After 6 months there was significantly less pain, inactivity stiffness, joint-line tenderness, and effusion in the 90Y-injected knees than in the controls (p less than 0.01). There were also significant differences between 90Y-injected and control knees in the changes in range of movement (p less than 0.01) and joint circumference (p less than 0.05) caused partly by progression of disease in the control knees. No significant differences in joint deformity, instability, X-ray appearance, or synovial-fluid analysis were seen. In all cases patient and observer assessment favoured the treated side (p less than 0.01). These findings indicate that intra-articular 90Y may be of benefit in chronic pyrophosphate arthropathy, a disease for which there is no treatment. The predilection of this condition to affect the knees of the elderly makes such treatment highly suitable because the joint lends itself readily to injection and the procedure carries very few actual or potential risks in this age group.