Objective To assess the effectiveness of local delivery of metronidazole as an adjunct to scaling and root planing (SRP) versus SRP alone in patients with chronic periodontitis. Methods We searched six major electronic databases including CNKI, PubMed, EBSCO, OVID, ScienceDirect, and Springrlink from establishment to May 2009. Randomized controlled trials (RCTs) comparing metronidazole plus SRP with SRP alone for at least 1 month of follow-up were included. The methodological quality of included trials was assessed by the method recommended by the Cochrane Collaboration and pooled analysis was conducted using Stata 10.0 software. Results Seven RCTs met the inclusion and exclusion criteria. A significant mean reduction in probing depth with WMD –0.43 and 95%CI –0.72 to –0.13 for the combined metronidazole and SRP was observed, but there were no statistical significant differences at the clinical attachment level between the two treatments with WMD –0.23 and 95%CI –0.53 to 0.06. Conclusion Metronidazole as an adjunct to SRP could reduce probing depth in the treatment of chronic adult periodontitis.