The present paper deals with a case of Pseudomyxoma peritonei arising from the appendix in a Brazilian, white, 58 years old, male. The growth amply spreads over the peritoneum, giving rise to strong adhesions amongst the peritoneal viscera, particularly on the part of the liver, spleen and pancreas (Ills. 4, 5 and 6). In certain spots, the growth even forms a mass the size of a foetus head, made up of a great number of blisterlike formations, well delimited by septa of connective tissue and full of a substance the appearance and consistency of the white of egg (Ill. 1). On microscopical examination, the majority of the formations present smooth walls; in some of them an epithelial covering is met with, the cells of which are cylindrical and ranged along a single layer (Ills. 2 and 3). The contents of the formations give the positive reactions of mucicarmine. The tumour stroma is infiltrated by mononuclear cellular elements, particularly by lymphocytes (Ill. 7); there are also found giant cells the type of those of foreign body (Ills. 8 and 9). In accordance with the anatomicopathological aspects met with, we consider the case as pseudomyxoma peritonei ex processu vermiformi, the same having to be classified in Group I of Trotter's systematization (dissemination of the tumour all over the peritoneum).