Behcet's disease is a clinical entity with mouth and genital ulcers, skin lesions, and both anterior and posterior uveitis as its major criteria. It has been theorized that Behcet's disease is immune complex mediated, and is characterized by multiple attacks which often lead to severe visual handicap. Behcet's disease patients with severe ocular involvement and cytotoxic and/or systemic corticosteroid agent failures were treated with cyclosporin (CsA). CsA therapy effectively abrogated the acute phase of the ocular attack and either totally prevented, or markedly reduced, the recurrences of these attacks. Generally, patients tolerated the medication well; occasionally however, renal toxicity secondary to cyclosporin therapy prevented prescription of a maximally effective therapeutic dose of the drug. CsA levels in plasma were dose-dependent for each patient, but the dosage per kg of CsA needed to obtain a specific plasma level varied greatly from patient to patient. Circulating immune complex levels were not universally elevated during the acute ocular attack. However, increases in circulating immune complex concentrations were noted after treatment with CsA was begun and the disease became clinically inactive. These data, as well as the effectiveness of CsA, an agent with predominantly anti-T cell effects, raise a question as to whether circulating immune complexes are centrally relevant to the pathogenesis of this disorder, and if T cell mediation of this disease must be contemplated.