Fifty chronic adult periodontitis patients completed a 6-week controlled, double-blind, split mouth clinical study to determine the effects of subgingival irrigation with an antimicrobial mouthrinse on periodontal microflora, supragingival plaque, and gingivitis when used as an adjunct to normal oral hygiene. Qualifying subjects had at least four sites, two on each side of the mouth, with probing depths between 4 and 6 mm, which bled on gentle probing. Following baseline examinations, subjects received a half mouth scaling and prophylaxis and full mouth subgingival irrigation with either the antimicrobial mouthrinse or sterile colored water control professionally delivered. Subjects continued irrigation at home once daily for 42 days with their assigned rinse delivered via a subgingival delivery system. All sites in the mouth were scored at baseline and at day 42 for supragingival plaque, bleeding on probing, and redness. For the four selected periodontitis sites, probing depth and attachment level were measured at baseline and on day 42; additionally, supragingival plaque and gingival redness were scored on days 7 and 21. Subgingival plaque samples for microbiological analysis were harvested from the selected periodontal sites at baseline and on days 7, 21, and 42. Microbiologically, irrigation with the antimicrobial mouthrinse resulted in statistically significant reductions compared to control in putative periodontopathogens, including black pigmenting species, which persisted at 42 days. Clinically, subgingival irrigation with the antimicrobial mouthrinse produced a significant reduction in supragingival plaque (P < 0.001), bleeding on probing (P = 0.019), and redness (P = 0.017) compared to the control, whether or not the area irrigated received a prophylaxis.(ABSTRACT TRUNCATED AT 250 WORDS)