OBJECTIVE Several recent clinical studies have described an association between HIV seropositivity and lung cancer. The purpose of this study was to describe the spectrum of imaging findings in HIV-positive patients who had proved carcinoma of the lung. In particular, we attempted to define the role of CT in diagnosing HIV-associated lung cancer. METHODS The study population consisted of 23 HIV-positive patients from two institutions who had lung cancer diagnosed between 1989 and 1993. All patients had both chest radiographs and CT scans. The group included 19 men and four women with a mean age of 42 years. The diagnosis of lung cancer was confirmed by bronchoscopy in eight patients, by pleural fluid aspiration or pleural biopsy in seven, by percutaneous needle biopsy of a lung lesion in three, by biopsy of an extrathoracic site in four, and by thoracotomy in one. Two thoracic radiologists retrospectively evaluated the chest radiographs and CT scans to identify parenchymal masses, lymphadenopathy, pleural disease, chest wall or mediastinal invasion, and metastatic lesions. RESULTS Fifteen (65%) of the 23 patients had a central or peripheral mass or nodule. Eight (35%) had extensive pleural disease, either as an isolated finding or in combination with other abnormalities. CT scans showed the malignant lesion underlying the extensive pleural disease in all but one case. All patients with extensive pleural disease had adenocarcinoma. No patient in the study was considered a candidate for resection on the basis of clinical and radiologic evaluation. CONCLUSIONS Lung cancer in HIV-positive patients manifested most often on chest radiographs as a central or peripheral mass or nodule. Extensive pleural disease in the absence of an apparent primary lesion was the second most common major manifestation. Lung cancer therefore merits serious consideration in the differential diagnosis of extensive pleural disease in HIV-positive patients. CT was most useful in evaluating malignant lesions associated with extensive pleural disease.