The cemental surface of teeth affected by juvenile periodontitis was examined using scanning electron microscopy. Molars affected by chronic marginal periodontitis and healthy molars were included as controls. The entire mesial root of teeth affected by juvenile periodontitis showed extensive areas of cementum hypoplasia with exposed dentinal tubules. The hypoplasias were found also on the infracrestal part of the root indicating that they had not been caused by exposure to the oral environment or by any treatment. Cementum hypoplasias were not found in any teeth from the control material. The impaired periodontal attachment in the hypoplasia may facilitate penetration by the special motile bacteria which have been found in plaque from teeth with juvenile periodontitis. Based on the findings in the present study and the hereditary background of juvenile periodontitis it was suggested that development of the disease is initiated by a hereditary developmental disturbance of the cementum.