Any irritation or disruption to the underlying bone will cause a periosteal reaction and result in new periosteal bone deposition. Periosteal bone formation may be due to either physiologic or pathologic causes. Pathologic bone formation generally results from an adjacent inflammatory process or a hypoxic or toxic stimulus. Common causes of pathologic periosteal reaction in children include trauma to the underlying bone. However, other causes such as hypervitaminosis A, prostaglandin therapy, cortical hyperostosis (Caffey's disease), hypertrophic osteoarthropathy (primary and secondary), osteomyelitis, leukemia, trauma, and syphilis must also be considered. The last four are usually associated with some degree of bone destruction, while in the first four diseases the underlying bone is left radiologically intact. This paper will concentrate on those diseases that appear to leave the underlying bone intact. The clinical and radiological features that help to differentiate some of these entities are presented.