A multifactorial morphologic study was performed in 72 children (49 boys and 23 girls) aged from 2 to 16 years, operated on because of osteoid osteoma and osteoblastoma of 12 anatomical sites. It is shown that different variants of early stages of osteogenic tumours correspond to different levels of cell differentiation and atypia. The smallest foci of dysplasia found near the tumour should be considered as pretumorous bone dysplasia.