A prospective analysis of 155 patients with pulmonary embolism was undertaken to describe the radiographic characteristics of associated pleural effusions and related abnormalities. Approximately one half of these patients had pleural effusions. Patients with other potential causes of effusion, such as heart failure, pneumonia, or cancer, were eliminated from further analysis. In the remaining 62 patients, radiographic evidence of pulmonary infarction accompanied pleural effusions in one half of the cases. One third of patients with parenchymal consolidation had no evidence of effusion. Atelectasis and other nonspecific radiographic abnormalities occurred in less than one fifth of the cases. Typically, pleural effusions were small and unilateral, appeared soon after symptoms of thromboembolism began, and tended to reach their maximal size very early in the course of the disorder. Pulmonary infarction was associated with larger effusions that cleared more slowly and were more often bloody in appearance on thoracentesis. Chest pain occurred in all but one patient and was a valuable diagnostic clue. Pain and pleural effusions were always ipsilateral and almost always unilateral, but neither correlated well with the presence or time course of infarction. Effusions that were delayed in onset or that enlarged late in the course were associated with recurrent pulmonary embolism or superinfection. These radiographic features may be helpful in the diagnosis and management of pulmonary embolism.