Intrusion has been regarded as a very controversial topic in the orthodontic literature. Although it seems a logical way to handle deep overbite in adult patients who have elongated teeth, reports on iatrogenic damage have led to the suggestion of alternative methods. Considering the disadvantages of these alternatives, it seems reasonable, however, to improve our knowledge of tissue reaction as related to intrusion. Three Macaca fascicularis monkeys were used for the experiment. By means of a segmented arch approach, the upper incisors and the four first premolars were submitted to forced eruption for 8 weeks followed by 12 weeks of intrusion. A split-mouth technique was used to study the influence of oral hygiene on the tissue reaction. On the right side of the mouth, the teeth were brushed with chlorhexidine three times per week. On the left side, no oral hygiene was performed. After intrusion of the teeth, a 1-to-14 day retention period with passive appliance preceded the killing of the monkeys. A buccolingual hematoxylin- and eosin-stained serial section was produced, and soft- and hard-tissue reactions described. It appeared that the hygiene program could limit but not prevent gingival inflammation. There was, however, a marked difference in the histologic picture of the marginal bone on the two sides. On the hygiene side, clear signs of bone deposited during forced eruption were still present. This was not the case on the nonhygiene side. The extension of bone resorption was also different on the two sides.(ABSTRACT TRUNCATED AT 250 WORDS)