It is not generally recognized that patients infested withStrongyloides stercoralismay have intestinal malabsorption, although this has been demonstrated by roentgenologic techniques. A heavily infested patient manifesting this symptom suffered a lethal reaction to a small dose of dithiazanine iodide. Usually this cyanine dye is sparingly absorbed, and serious toxicity has not been reported during its clinical trials. At necropsy, however, this patient showed evidence of dye absorption in the intestinal lymphatic channels and staining of the viscera. This unexpected reaction may be explained on the basis of phagocytosis of dye particles by the inflamed intestinal mucosa, the process being enhanced by slow transit of the dye through the upper intestine. It is recommended, therefore, that malabsorption be ruled out before giving this drug to patients with heavy infestations of this type.