AIMS--To describe the birefringent saponified fatty acid crystalloids seen in pancreatic fat necrosis. METHODS--A histological review, including polarising microscopy, of three cases of subacute or subclinical acute pancreatitis was performed. Histochemical analysis using Nile blue sulphate for lipid, Holczinger's copper rubeanate for fatty acids, and Alizarin Red S for calcium was performed in one case. Scanning electron microscopy and x-ray energy dispersive spectroscopic micro-analysis were performed in two cases. Necropsy pancreatic tissue, surgical archival tissue from cases of pancreatitis, and pancreatic and adipose tissue permitted to autolyse together in the laboratory, were also examined. The autolysed tissue was also examined histochemically. Stained and unstained sections were mounted in DPX and Canada balsam. Surgical material showing traumatic fat necrosis was reviewed. RESULTS--In each of the three cases there were subtle clues to subclinical pancreatitis. In neither surgical case was the true nature of the mass apparent to the operator. Histological analysis in all cases showed ghost adipocytes containing numerous polarising crystalloids, as well as some basophilic debris. Microanalysis showed calcium but no other substantial heavy element signals. Histochemical analysis showed a labile, polar, acidic lipid and the crystalloids behaved as calcium salts of free fatty acid. The crystalloids were not seen in archival material mounted in Canada balsam. No crystalloids were seen in traumatic fat necrosis. CONCLUSIONS--Little recognised, strongly birefringent, saponified free fatty acid crystalloids occurring in pancreatic fat necrosis may survive routine processing, and can point to the origin of obscure mesenteric masses related to subclinical pancreatitis.