This article reviews the life-threatening fluid and electrolyte abnormalities that develop in association with, or as a result of neoplasms or their therapy. Ectopic hormone secretion by tumors and their resultant electrolyte aberrancies are also discussed. The emphasis of this article is on clinical phenomena encountered in the ICU that are specific to cancers and their therapy rather than being a review of electrolyte abnormalities in general. Each topic includes a discussion of the management of the abnormality.