One hundred three relapsed leukemia patients were treated with high-dose cytosine arabinoside (Ara-C); 3 g/m2 intravenously over 2 hours every 6 to 12 hours for a total of nine to 12 doses or 3 g/m2 intravenously over 2 hours for two doses 12 hours apart followed by a continuous infusion of 1.5 g/m2 over 24 hours daily for 3 to 4 days. Thirteen of them developed adult respiratory distress syndrome (ARDS) without having any recognized reason for the development of pulmonary edema. This problem showed no correlation with age or prior chemotherapy. Four of the patients recovered, but in nine this complication was fatal. The authors have reviewed the clinical course of these 13 patients and the postmortem findings of the seven patients who had an autopsy performed. The pulmonary tissue from six patients showed massive edema and one had diffuse alveolar damage. Histologic examination revealed a highly proteinaceous intraalveolar infiltrate without any inflammatory reaction in all cases. Intestinal tissue from all patients revealed changes compatible with cytotoxic damage, and pleura and/or pericardium from six of the seven patients showed an extensive fibrinous exudate suggestive of capillary leakage. The time sequence of the clinical events and the histologic findings indicate that high-dose Ara-C treatment in leukemia may cause a capillary leakage syndrome with ARDS that may progress to fatal respiratory failure.