Abstract The relationship between the radiological and histological findings was studied in a series of rabbit osteotomies and fractures stabilized with plates with and without longitudinal compression. When compression was employed, the gap varied between 10 and 200 μm and no radiologically visible callus was seen. After 4 weeks the smallest gaps of 10–30 μm were filled histologically with structureless material, while the larger gaps contained ‘cortical’ bone derived from periosteal and endosteal proliferation. The degree of union could not be assessed from X-ray examination, as there was no correlation between the histological and radiological appearances. When no compression was employed, the gap varied between 200–1200 μm; radiologically visible periosteal callus and filling of the gap indicated that consolidation had appeared in four weeks. Gaps of 200–500 μm were filled with bone derived from the periosteum and endosteum of lamellar structure which had invaded the original cortical bone (‘plugging’). When the gaps were more than 500 μm, islands of cartilage and connective tissue were found. In all cases studied, many ‘anchoring osteons’ passed from the endosteal and periosteal callus into the original cortical bone. It is concluded that a ‘compression internal fixation’ of cortical bone will not necessarily secure union, as histologically confirmed, even when the formation of radiologically visible callus is suppressed. The lack of periosteal callus appears to be the result of a small gap and rigid fixation, rather than related to strains induced in the bone.