To evaluate prospectively the clinical features, angiographic findings, and response to treatment of patients with Takayasu arteritis. 60 patients with Takayasu arteritis were studied at the National Institute of Allergy and Infectious Diseases between 1970 and 1990 and were followed for 6 months to 20 years (median follow-up, 5.3 years). Data on clinical features, angiographic and laboratory findings, disease course, and response to therapy were all recorded and stored in a computer-based retrieval system. The Warren Magnuson Clinical Center of the National Institutes of Health. In our series of patients, Takayasu arteritis was more common in Asian persons compared with persons from other racial groups. Females (97%) were most frequently affected. The median age at disease onset was 25 years. Juveniles had a delay in diagnosis that was about four times that of adults. The clinical presentation ranged from asymptomatic to catastrophic with stroke. The most common clinical finding was a bruit. Hypertension was most often associated with renal artery stenosis. Only 33% of all patients had systemic symptoms on presentation. Sixty-eight percent of patients had extensive vascular disease; stenotic lesions were 3.6-fold more common than were aneurysms (98% compared with 27%). The erythrocyte sedimentation rate was not a consistently reliable surrogate marker of disease activity. Surgical bypass biopsy specimens from clinically inactive patients showed histologically active disease in 44% of patients. Although clinically significant palliation usually occurred after angioplasty or bypass of severely stenotic vessels, restenosis was common. Medical therapy was required for 80% of patients, whereas 20% had monophasic self-limiting disease. Immunosuppressive treatment with glucocorticoids alone or in combination with a cytotoxic agent failed to induce remission in one fourth of patients; about half of those who achieved remission later relapsed. In North America, Takayasu arteritis is a rare disease. It is heterogeneous in presentation, progression, and response to therapy. Current laboratory markers of disease activity are insufficiently reliable to guide management. Most patients require repeated and, at times, prolonged courses of therapy. Although mortality was low, substantial morbidity occurred in most patients.