The aim of this study was to evaluate the clinical and bacteriological effects of the intrasulcular application of a 1% metronidazole-gel (repeated administrations outdistanced of 7 days weeks long) currently employed in dermatological practice, to observe if a lower concentration of the chemotherapic agent could be equally effective as the 25% formulation in improving the periodontal condition of nine patients with adult periodontitis. The results showed that this regimen can modify, at a statistically significant level, the clinical (Pocket Probing Depth, Gingival Bleeding Index and Plaque Index) and bacteriological (Actinobacillus actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia and Veillonella parvula) parameters associated with adult periodontitis. The results are similar to those obtainable with a 25% Metronidazole-gel administered two times outdistanced by 7 days.