The aim of this prospective clinical 2-year study was to determine whether the placement of the proximal margins of crowns within the zone of the biologic width results in periodontal alterations. In 41 patients, 116 prepared teeth as well as 82 unrestored, healthy contralateral teeth were examined. The following indices were determined before as well as 3, 6, 12, and 24 months after preparation: hygiene index, papillary bleeding index (PBI), and probing depth. After preparation, the distance between the restoration margins and the alveolar crest was measured with a modified periodontal probe on both proximal aspects of each tooth. These values were classified into 3 groups: I = < or = 1 mm between crown margin and alveolar crest, II = 1 to 2 mm, and III = > 2 mm. In addition, radiographs were taken directly after cementation of each restoration and after 12 and 24 months. The hygiene index did not significantly vary during the investigation, but PBI increased in all groups. The highest PBI increase was found in group I; in this group, the most significant increase was found between 3 and 6 months after preparation. The mean probing depth only increased in group I during the first 3 months after preparation on the mesial aspects. Only minor changes were found on the distal aspects of the teeth in group I and on all aspects in groups II and III. No alterations of the bone levels were diagnosed on the radiographs. These results indicate that the location of the restoration margins within the zone of the biologic width may impair the periodontal health of restored teeth.