Aims: Cell enlargement or hypertrophy is an intermediate transitional process in the transformation of Òsoft callusÒ into bone. The purpose of this study was to determine whether it is caused by an osmotic phenomenon. Following bone fracture, there is a local increase in tissue ßuid due to inßammation and neovascularisation. According to the osmosis principle, cells bathed in excess tissue ßuid swell. Methods: The specimens examined were 1 and 2-week old closed fractures of the right tibia of 12 NZW rabbits created by a drop tower technique. The specimens were prepared for routine histology. Thin sections were stained for haematoxylin and eosin and examined with the light microscope. Results: Cell enlargement and cell rupture were observed principally in the vicinity of the blood vessels. There was a hierarchy of cell sizes with the larger cells close to the blood vessels and the smaller ones further away from the vessels. Conclusions: According to these þndings, the fracture callus exhibits features which raise the possibility that an osmotic phenomenon is responsible for cell enlargement. The resulting increase in cell turgidity makes the fracture callus progressively stiffer and increases tissue strain. Cell enlargement also causes the tissue to expand. This may be the mechanism by which fracture callus migrates and bridges the fracture cleft.