A 33-year-old white female of English descent with beta thalassaemia trait developed painful recurring bilateral knee effusion at age 15 years. Trauma was denied. Synovial analyses revealed noninflammatory effusions with normal complement, no inclusions, and no crystals. Knee x-rays normal at ages 18 and 26, showed mild osteoarthritic changes at age 33. Laboratory tests for other known causes of arthritis were repeatedly normal or negative. Bone densitometry was below normal. Light microscopy of the synovial membrane showed no significant abnormalities and no iron deposition. Electron microscopic findings included multilamination of vascular basement membranes and large amounts of thin fibrils surrounding many connective tissue cells. Treatment with salicylates did not prevent recurrence of effusions, and quadriceps strengthening and joint rest were moderately successful in relieving pain. Intra-articular corticosteroids on 2 occasions were not helpful. Whether her knee arthritis is purely secondary to the para-articular bone thinning from the chronic marrow expansion remains to be determined.