This article presents the results of a single-arm, open-label, multicenter clinical trial of the topical use of sustained-release minocycline hydrochloride (HCl) microspheres as an adjunct to scaling and root planing. The objective of this study was to evaluate the long-term safety and efficacy of the subgingival application of resorbable minocycline microspheres as an adjunct to scaling and root planing in the treatment of chronic periodontitis. The primary outcome measures were the reduction in probing pocket depth at 9- and 12-month evaluations, and the percent of bleeding upon probing. A total of 173 patients with moderate-to-severe chronic periodontitis were enrolled in this multicenter clinical trial. All patients received full-mouth scaling and root planing plus minocycline microspheres in all periodontal pockets that probed > or = 5 mm. All sites treated at baseline and any new sites > or = 5 mm again received minocycline microspheres at 3- and 6-month follow-up appointments with no further scaling and root planing. Significant improvements in all clinical parameters measured were found at all time points (1, 3, 6, 9 and 12 months). The product was found to be well-tolerated by patients, safe, and easy to deliver. Scaling and root planing with the topical application of minocycline microspheres appeared to give better results than would have been expected with scaling and root planing alone.