We conducted retrospective chart and literature reviews to analyze the frequency and spectrum of causes of asymptomatic pleural effusion (APE). In our series, 16 percent of patients undergoing thoracentesis for PE were asymptomatic and the spectrum of causes was similar to that for symptomatic patients. Asymptomatic PEs were evenly distributed among transudates, exudates and indeterminate effusions. More symptomatic (S) PE were exudates, although the difference was not statistically significant (p greater than 0.1). In comparison to SPE, APE were more often free flowing and small. In both groups, the four most common diagnoses were malignancy, CHF, parapneumonic and postoperative effusions accounting for greater than 70 percent of each group. Review of the literature demonstrated the following associations with APE: recent childbirth or abdominal surgery, benign asbestos effusion, uremia, malignancy, and tuberculosis. In the uncomplicated postpartum or postoperative setting or in patients with typical findings of left ventricular failure, observation without diagnostic studies is appropriate. In all other situations, APE should be evaluated in traditional fashion. If thoracentesis is non-diagnostic and the effusion is an exudate, closed pleural biopsy and less often, fiberoptic bronchoscopy, should follow. Once malignant or granulomatous pleuritis has been excluded, it may be appropriate to observe for a period of time before proceeding to more invasive procedures.