The clinical and immunologic findings in 10 patients with Behçet's disease are described. The frequency of inflammatory lesions was aphthous stomatitis, nine patients; ocular inflammation, six; genital ulceration, eight; arthritis or arthralgias, seven; cutaneous lesions, six; nervous system involvement, four; and phlebitis and ulcerative colitis, three each. Previously unreported complications encountered singly were pancreatitis, peripheral neuropathy, subungual infarctions, and malignant lymphoma. All four biopsy specimens of mucocutaneous lesions showed vasculitis, in one associated with fibrinoid necrosis. Polyclonal immunoglobulin increases were lgA, four of nine; lgM, four of nine; and lgG, one of nine. Anticytoplasmic antibodies to human cadaver esophagus cells were present in all five tested. Search for viral inclusions and isolations proved negative. Corticosteroid treatment, although reducing painful inflammation, did not prevent neurologic progression in one patient or uveitis in another. A previously reported improvement after blood transfusion was noted in two of three patients.