Interleukin-2 is a glycoprotein physiologically produced by human lymphocytes which is capable of mediating some still unknown immunologic reactions. In vitro, interleukin-2 was seen to induce a lytic reaction against tumor cells through the activation of a cytolytic system of natural killer cells. If administered to man in heavy doses, it causes a clinical response in the treatment of metastases from melanoma and renal cell carcinoma in 20-40% of cases. However, the clinical use of the drug, in therapeutic doses, is prevented by the occurrence of several side-effects, the major one being increased permeability of alveolar vessels with capillary leak and interstitial pulmonary edema (Vascular Leak Syndrome in the English literature). Thus, this work was aimed at evaluating chest radiographs during interleukin-2 treatment to detect, in the pulmonary district, the early stages of the vascular leak syndrome--i.e., pulmonary edema, pleural and pericardial effusions. Forty-three patients had been treated for metastases from renal cell carcinoma and melanoma November 1989 through September 1991: standard chest radiographs demonstrated 26 cases (60%) of pulmonary edema, 14 cases (32%) of bilateral pleural effusions and 12 cases (27%) of pericardial effusions. Daily chest films of the patients undergoing interleukin-2 therapy allowed the early stage of the vascular leak syndrome to be depicted, thus enabling the physician to use the highest tolerated doses and eventually to stop infusion before marked respiratory distress develops.