This investigation is the result of a multicenter research project between the Catholic University of Leuven and the University of Madrid with the purpose of studying histometrically and ultrastructurally periimplant marginal tissue reactions in mendium-term Brånemark implants. 12 patients were selected and distributed according to predefined clinical and radiological parameters into a periimplant infection group (A) and a non-periimplant infection group (B). Biopsies were taken from these patients and analyzed by means of microscopy (LM), histometry (HM) and transmission electron microscopy (TEM). With LM group A showed pathological features consistent with an advanced gingivitis lesion, while group B showed signs of healthy gingiva in the connective tissue and vauolae formation in the epithelium. With HM, group A showed significantly higher transmigration of inflammatory cells in the epithelium than group B. In the connective tissue, group A showed a higher % of inflammatory infiltrate with significantly higher numbers of plasma cells and mononuclear cells than group B. With TEM, group A showed an ultrastructural picture consistent with plaque-induced periodontitis, while group B showed a rather healthy connective tissue and vacuolae formation in the epithelium. These results suggest that supracrestal marginal tissues around osseointegrated implants react to plaque bacteria similarly to periodontal tissues around teeth by chronic inflammation.