A case of fatal adhesive or proliferative arachnoiditis resulting from particles of surgical cotton (cottonoid1) left at operation prompted the study of which this paper is a report. Because of the observations in this case we have reexamined all of the material we had available in cases in which autopsy had followed operation or in which a previous operation had been done, and in nearly every instance we found that occasional cottonoid fibers had been left and that about them there was a foreign body reaction of a granulomatous type. In most of the specimens the amount of cottonoid was scant and the amount of tissue reaction so slight that it could hardly have been of clinical significance. Yet we feel that in many instances some fibers of cotton or cottonoid left in the wounds, depending on the amount and location, are capable of producing a reaction that will