Percutaneous needle aspiration biopsies of the lung from five patients with Wegener's granulomatosis were reviewed. Three of the patients presented with the generalized form of the disease while two presented with the limited pulmonary form; one of the latter subsequently developed disseminated disease. The morphologic findings in the pulmonary aspirates were similar in all cases. The cytologic preparations contained neutrophils entrapped within necrotic debris plus scattered but prominent histiocytic giant cells, which often had nuclei arranged in rings or horseshoes, in a background of lymphocytes, epithelioid histiocytes and reactive pneumocytes. Cell block preparations showed discrete areas of necrosis containing a neutrophilic infiltrate and focally palisaded by epithelioid histiocytes. The intervening viable tissue contained prominent histiocytic giant cells and chronic inflammatory cells enmeshed in a fibrous matrix. One cell block contained a small artery with a small focus of possible granulomatous arteritis. While an open lung biopsy is generally required for a definitive diagnosis, the pathologist may encounter unsuspected Wegener's granulomatosis in a needle aspirate. Recognition of the findings observed in these cases should alert the pathologist to the possibility of Wegener's granulomatosis so that an open lung biopsy can be performed if clinically indicated and cytotoxic therapy can be promptly instituted if the diagnosis of this entity is confirmed.