A 54 year old man presented with a six month history of abdominal pain. A computerised tomography scan showed a well defined intra-abdominal unilocular mass with a calcified wall just superior to the bladder. At laparotomy, pseudomyxoma peritonei was discovered, together with a midline abdominal mass adherent to the anterior abdominal wall originating from the fundus of the bladder. The specimen consisted of a cystic mass measuring 14 x 9.5 x 7 cm overall, which contained mucoid material. Histological examination revealed that the cyst was lined by mucinous epithelium, which in areas varied from having bland morphology to showing pronounced nuclear and architectural atypia. There was abundant extracellular mucin. The specimen was extensively sampled but there was no evidence of invasion. This tumour has many unusual features, namely: the absence of destructive invasion, association with pseudomyxoma peritonei, areas of dysplasia and cystadenoma, and stromal osseous metaplasia within the wall.