This study compared oral irrigation and rinsing with chlorhexidine (CHX) and placebo in the treatment of naturally occurring chronic gingivitis. 44 subjects with at least 6 interproximal sites which bled on probing were randomly distributed on a double-blind basis into 4 treatment groups, placebo-rinse, CHX-rinse (0.12%), placebo-irrigation and CHX-irrigation (0.06%). A half-mouth was scaled 2 weeks prior to therapy in all groups. Rinses were performed 2 times daily and irrigation was performed once a day by means of an oral irrigator with the tip directed at a right angle to the tooth. Subjects continued with routine oral hygiene without instruction. The active treatment period was 2 months. Parameters were recorded at baseline and at 60 days. At the conclusion, marginal plaque was cultured for predominant microbial types. CHX-rinse (0.12%) and CHX-irrigation (0.06%) significantly reduced (p less than 0.05) plaque. Gingival bleeding decreased by 26% in both scaled and unscaled sites following CHX (0.12%) rinses and by 40% at both types of sites following CHX (0.06%) irrigation. Bleeding was reduced with CHX-irrigation greater (p less than 0.05) than with the placebo-irrigation. The mean log of colony-forming units of Actinomyces species was significantly lower (p less than 0.05) in the CHX (0.12%) rinse and CHX (0.06%) irrigator groups than in the placebo groups. These data therefore indicate that delivery of CHX (0.06%) by an oral irrigator is an effective means of treating naturally occurring gingivitis.