The radiographic abnormalities of the interphalangeal joint of the great toe in 50 patients with rheumatoid arthritis were compared with those in 14 patients with psoriatic arthritis. Alterations were seen in 50% of the rheumatoid joints. Characteristic superficial erosions along the medial aspect of the proximal and distal phalanges, with little change in the joint space, differed from more widespread and extensive destruction of this joint in psoriasis. In addition, periosteal proliferation, bony ankylosis and tuft resorption, frequently noted in psoriatic arthritis, were unusual in rheumatoid disease. An anatomic investigation of the interphalangeal joint of the great toe explains the sites of marginal erosions in rheumatoid arthritis. Although selective involvement of this articulation occurs in psoriatic arthritis and Reiter's syndrome, it is the radiographic pattern of articular abnormality which allows their differentiation from rheumatoid arthritis.