In 5 monkeys a total of 30 transmucosal endosseous dental implants were inserted in edentulous areas of the mandible with presence or absence of keratinized mucosa. After a healing period of 3 months with optimal plaque control, all implants were exposed to plaque accumulation for periods up to 9 months. To secure abundant plaque accumulation on half the number of the implants, cotton wool ligatures were placed around the implants at the entrance to the peri-implant sulcus. Attachment loss was measured clinically and histometrically, and tissue recession was measured clinically. Ligated implants without keratinized mucosa demonstrated significantly more recession and slightly more attachment loss than the other implants. The results of this study suggest that the absence of keratinized mucosa around dental endosseous implants increases the susceptibility of the peri-implant region to plaque-induced tissue destruction.