The periodontal condition in 53 adolescents undergoing long–term orthodontic treatment with fixed appliances was examined longitudinally for 4 years. In each case measures aimed at establishing a proper oral hygiene standard were carried out prior to and during active orthodontic treatment. Sixteen adolescents matched with regard to age and free from orthodontic treatment served as controls. The results showed that multiband orthodontic treatment led to a slight but significant loss of periodontal support despite good or excellent individual oral hygiene. At the end of 24±3.5 months of active orthodontic treatment the average loss of buccal and lingual attachment for banded teeth amounted to 0.28 mm and 0.22 mm, respectively. A mean reduction of 0.29 mm of interproximal marginal bone height was found. These alterations were irreversible but no further deterioration was observed during a subsequent retention and follow-up period of 20 ±3.0 months. A detailed analysis of the periodontal condition of the second premolars indicated that the changes occurring adjacent to the different tooth surfaces could not be explained solely by the presence of dental plaque. Extraction of neighbouring first premolars and tooth movements during active orthodontic treatment may also have played a role.