Forty-three pleural fluids from 30 patients with eosinophilic pleural effusion (EPE) were examined and correlated with biochemical and clinical findings. The percentage of eosinophiles ranged from 12% to 83%, averaged 38% and was accompanied by a high lymphocyte count, averaging 34%. The fluids were shown to be exudates. A history of allergy or immune disorders was elicited from seven patients and allergy to drug from one patient. Eight patients gave a history of thoracic trauma unrelated in time to the effusion. Three patients received transvenous pacemakers. In the remaining 11 patients the presence of the eosinophilic pleural effusion could not be correlated with any clinical data. Regardless of clinical setting, which in six patients included a past history of cancer, EPE proved to be a self-limiting disease with a favorable outcome. Thus, this diagnosis by cytologic examination of the sediment is of considerable prognostic significance even in patients with a history of other disorders that may give rise to effusions. Possibly the most significant observation recorded was the presence of a large population of lymphocytes in EPE. The role of these cells in the mechanism of EPE is discussed.