Giant cell arteritis is a generalized inflammatory disorder involving large and medium-sized arteries. The etiology is unknown, although an autoimmune pathogenesis seems probable. In view of the clinical similarities between patients with positive biopsy findings for polymyalgia rheumatica and those with negative biopsy findings, many authors favor the concept that polymyalgia rheumatica is an expression of an underlying giant cell arteritis. There is, however, still controversy as to whether polymyalgia rheumatica and temporal arteritis are different expressions of one and the same disease or two separate, partly overlapping types of giant cell arteritis. A single etiologic factor may be responsible for the two conditions, sometimes expressing itself as polymyalgia rheumatica and sometimes as giant cell arteritis. Recent findings of morphologic similarities in terms of arterial wall atrophy, calcifications, and inflammatory reactions may indicate that polymyalgia rheumatica and temporal arteritis represent different degrees or stages of the same disease.