The great frequency with which Dr. MacMillan, roentgenologist of the Massachusetts Eye and Ear Infirmary, is able to demonstrate the presence of cysts in the maxillary sinus by x-ray examination aroused my curiosity and caused me to undertake an investigation into their formation and structure from a pathologic standpoint. This work was carried out in Dr. Mosher's laboratory at the Harvard Medical School. I had access to all the specimens at the Massachusetts Eye and Ear Infirmary and to Dr. Kazanjian's private collection of dental cysts. In this paper, it is my purpose to deal only with the common types of cysts, namely, secreting cysts, dental cysts of inflammatory origin and mesothelial cysts, omitting the dentigerous types which are rather rare and which are concerned with maldevelopment of the tooth follicle, and also those belonging to the group of cystic adamantinoma. The latter are essentially new growths rather than cysts.