Infantile cortical hyperostosis, also known as Caffey's disease or Caffey-Silverman syndrome, is an uncommon clinico-pathological lesion of unknown etiology and uncertain histogenesis. One of the most striking features is the early age of patients at the onset of the disease, showing swelling of the soft tissues overlaying bones, hyperirritability, and, subsequently, periosteal new bone production. The natural history of the disease proves to be self-limiting. Multiple areas are involved in the majority of cases. These polyostic forms are easily clinically diagnosed. But in rare monostic presentations, especially manifestations in the scapular region, there may be a great suspicion of a malignant tumor. Histologically, such lesion may also be misdiagnosed as a malignant neoplasm because of the great variety of microscopic appearances. This study was conducted into 5 cases (biopsies from one male and four female infants, 6 weeks to 4 months of age) to characterize the histological variability in the natural course of the disease. Electron microscopical investigations were additionally performed on two cases. Histologically, the process corresponds to typical ossifying periostitis. Three phases can be distinguished according to the main histological characteristics: 1. Acute inflammatory and proliferating phase; 2. Osteogenic phase; 3. Phase of remodelling. The first phase is characterized by a loss of periost, areas showing proliferation of fibroblast-like cells, and by edema of surrounding musculature. Infiltration by leucocytes was occasionally observed and was accompanied by micro-abscesses. The osteogenic phase was characterized by formation of woven bone. Ultrastructurally proliferations of osteogenic mesenchyma were found and resulted in typical mineralization patterns with matrix vesicles and interfibrillar depositions of hydroxyapatite crystals. Calcification of mitochondria was also detected. Viruses could not be observed. Only thread-like structures were found in the nuclei. At first interpretation, they appeared to be pathological protein depositions. However, further investigations will be necessary to elucidate their genesis. The pathogenesis is discussed. Biopsy still remains indicated in cases of an unclear course of monostic disease.