Hantavirus pulmonary syndrome (HPS), is a rodentborne, acute, often fulminant cardiorespiratory illness. Noncardiogenic pulmonary edema is prominent in HPS as is cardiac dysfunction. Pleural effusions are commonly noted in patients with HPS and have been thought to be exudative. This report describes the prevalence and characteristics of pleural effusions by an assessment of chest radiographs for the presence of pleural fluid and reviews all pleural fluid specimens obtained from patients with HPS. Of 23 patients treated at the University of New Mexico Hospital for HPS, 22 had evidence of pleural fluid while 4 had sampling of their pleural fluid. Two samples met criteria for an exudate by pleural fluid protein to serum protein ratio of more than 0.5; one was clearly a transudate and the other had inconsistent characteristics. The two exudative samples were obtained 7 days after admission, while the other 2 were obtained within 1 day of admission. Pleural fluid cultures were sterile, and the total of nucleated cells was less than 170/mm3, and predominately mononuclear. A hypothesis may be formulated that the pleural fluid in HPS is initially transudative, consistent with the observed cardiopulmonary dysfunction. However, following aggressive resuscitative efforts and as the acute illness resolves, fluid shifts occur as cardiac function normalizes; the pleural fluid may take on characteristics of an exudate.