Factor-VIII-related von Willebrand factor (vWF) multimers are synthesized by endothelial cells, and plasma vWF antigen levels are elevated in some disorders associated with endothelial cell perturbation. We studied 13 patients during cisplatin-basedcombination chemotherapy for squamous cell carcinoma of the head and neck, esophagus, or lung. Before therapy, 3 of the patients had vWF antigen levels that were ≥400% of normal; and further elevations occurred during chemotherapy. Two of these patients had cerebrovascular accidents, and the third had complications similar to the hemolytic-uremic and acute respiratory distress syndromes. No abnormalities in plasma vWF patterns were detected. Elevated plasma vWF antigen levels before therapy may identify a subgroup of patients at special risk for arterial occlusive complications following cisplatin-based chemotherapy.