Nine cases of pleural fluid infection caused by Listeria monocytogenes (one case described here and eight cases previously reported in the literature) were reviewed. Eight patients (88.9%) had an underlying malignancy (three had Hodgkin's disease, three had non-Hodgkin's lymphoma, and two had leukemia), and six (66.7%) were receiving immunosuppressive therapy at the time of presentation. Seven patients (77.8%) presented with fever and five (55.6%) with respiratory tract symptoms. Those with symptoms of greater than 3 weeks' duration had a relatively poor prognosis. Bacteremia was documented in five patients (55.6%). Examination of pleural fluid typically revealed normal levels of glucose, slightly elevated concentrations of protein, and a negative gram stain. Four patients died, for an overall mortality of 44.4%. Mortality appeared to be lower for patients who received a combination of penicillin or ampicillin plus an aminoglycoside and for those who underwent drainage of pleural fluid than for those not given such treatment. Rapid diagnosis, prompt institution of appropriate antimicrobial therapy, and drainage of the pleural fluid are likely to improve the chances for survival in listerial infection of pleural fluid.