This paper discusses differences between statistical and clinical significance. It suggests nine factors to consider when assessing clinical significance of periodontal therapy. The article reviews the multicentre randomized clinical trials on the four antimicrobial local delivery systems commercially available in the USA (tetracycline fibre, chlorhexidine chip, doxycycline gel and minocycline microspheres) and assesses their data in relation to the nine factors for clinical significance. Data from longitudinal studies comparing periodontal surgery to non-surgery were evaluated in a similar fashion to provide a frame of reference. It was concluded that the local delivery of an antimicrobial offers the clinician a statistically and clinically significant option in the treatment of chronic periodontitis. It is not an 'either/or' situation in selecting therapeutic methods. It is a choice of which treatment method works best for that patient by that clinician.