To the surgeon, fat necrosis is a familiar pathologic reality. It is a pathognomonic sign of rapidly destructive changes in the pancreas. In appearance it takes the form of small opaque white areas which are signposts of impending danger from hemorrhagic necrosis or other forms of acute fulminating infection of the pancreas. The recognition of these small chalky spots during an operation, therefore, becomes a matter of vital importance. They may be present but not apparent. When few and widely scattered they may easily escape recognition, especially if one does not have them in mind. My object in this communication is to stress the importance of searching for fat necrosis in all cases of acute infection of the gallbladder and bile ducts, and to point out a concealed area where fat necrosis may be found before it appears on the peritoneum ordinarily limited within the surgeon's range of view.