This study aimed to evaluate the utility of the pleural fluid lactate dehydrogenase (LDH) isoenzyme algorithm for the differential diagnosis of pleural fluid in patients with haematological malignancies. Twenty consecutive haemato-oncological patients with pleural effusion, hospitalized in the Haematology Department during a 2.75-year period, were prospectively and independently evaluated for the cause of effusion by standard methods and the LDH isoenzyme algorithm. The causes of the pleural effusions established during the standard evaluations were compared to the results obtained from the LDH isoenzyme algorithm. Following the standard evaluation, the pleural effusion was attributed to congestive heart failure in one patient, to infection in six, to the underlying malignancy in 12 and to concomitant congestive heart failure and malignancy in one. LDH isoenzyme analysis correctly predicted the cause of pleural effusion in 18 patients (positive predictive value 90%). In haemato-oncological patients, the pleural fluid LDH isoenzyme pattern may be helpful in the differential diagnosis of the most common causes of pleural effusion.