The purpose of this study was to evaluate the clinical effects of a single application of a 5% metronidazole collagen device in periodontal pockets deeper than 5 mm, in association with debridement and without reinforcement of home care and hygiene as practiced by the patient at any time. Pocket depth, attachment level, bleeding on probing, gingival index and plaque index were assessed at baseline, and on days 15, 30 and 90. Analysis of data from 28 patients indicated that both debridement and metronidazole therapy decreased pocket depth, bleeding on probing and gingival index, but results were significantly better with metronidazole. These results indicate that topical metronidazole provides an effective adjunctive treatment of advanced periodontitis.