Pseudomyxoma peritonei or gelatinous ascites is a rare clinical entity, and its pathogenesis remains obscure. It most often follows a mucinous tumor of the appendix. An ovarian origin in woman has been suggested but remains controversial. Its onset is often insidious: an increase in the abdominal perimeter may be the first sign noted. Preoperative diagnosis is facilitated by modern imaging techniques. Ultrasonography and computed tomography provide complementary signs: septa and scalloping of the liver margins, respectively. Effusion in the lesser peritoneal cavity suggests this diagnosis. Magnetic resonance imaging, by showing the gelatinous ascites, their septa and the scalloping of liver and spleen, can strengthen the probability of the diagnosis. Only laparotomy can confirm it, however. Appendectomy is required in all cases. Recurrence is more frequent in the forms associated with malignant or bipolar tumors. Cytoreductive surgery combined with hyperthermic intraperitoneal chemotherapy is the only therapy shown to be effective in cases of recurrence or malignant forms.