PURPOSE: This study attempts to determine the distribution of transudates vs exudates in pathologically proved malignant pleural effusions and the necessity for cytologic studies in patients with a transudative effusion. MATERIALS AND METHODS: A retrospective review of all cytologically positive malignant pleural effusions was performed at Duke University Medical Center over an 18-month period. All effusions were characterized as a transudate or an exudate based on standard criteria, including lactate dehydrogenase and protein values. RESULTS: Ninety-eight patients with a mean age of 62 years were identified as having a cytologically positive malignant pleural effusion and blood chemistry values available to distinguish an exudate from transudate. Ninety-seven patients (99%, 95% confidence interval; 0.94 to 0.99) had criteria for an exudative effusion. One patient (1%) with diffuse metastatic lung cancer had a borderline transudate and was in congestive heart failure at the time of thoracentesis. CONCLUSIONS: Cytologically positive pleural effusions for malignancy are almost always exudates. Cytologic evaluation for malignant cells of a transudative pleural effusion is not recommended.