The present investigation was undertaken to study the clinical effect of professionally performed periodic subgingival irrigation per se and as an adjunct to scaling and root planing. 10 patients suffering from moderate-severe periodontal disease participated in the study. Following an initial 3-month period of supervised supragingival plaque control, a total of 102 periodontal sites with probing pocket depth greater than or equal to 6 mm and "bleeding on probing" were selected and subjected to a Baseline examination comprising assessments of oral hygiene and gingival conditions, probing depths and probing attachment levels. The pockets in the various jaw quadrants were randomly assigned to one of the following treatment groups: (1) periodic subgingival irrigation with hydrogen peroxide, (2) periodic subgingival irrigation with chlorhexidine, (3) periodic subgingival irrigation with saline and (4) no subgingival treatment. During the first part of the study (baseline-32 weeks), no mechanical debridement of the subgingival area was performed. The irrigation treatment was carried out by the operator 3 times per week during weeks 1 + 2 and 5 + 6 of the trial. In the 2nd part of the trial (32-52 weeks), the sites were subjected to scaling and root planing combined with professional irrigation during weeks 32-38. The previously non-irrigated control sites were not subjected to adjunctive irrigation when mechanically debrided. During the entire study, the patients were recalled for professional tooth cleaning once every 4 weeks. Re-examinations were carried out at 4, 6, 32, 40 and 52 weeks. The results revealed that repeated professional irrigation of unscaled periodontal pockets with chlorhexidine or hydrogen peroxide resulted in a temporarily reduced frequency of bleeding sites, but not in any clinically significant changes in probing assessments. A similar improvement of bleeding scores was observed in the saline-irrigated control group. Scaling and root planing, in combination with an optimal supragingival plaque control, resulted in a marked resolution of the clinical symptoms of periodontal disease. Adjunctive irrigation with chlorhexidine or hydrogen peroxide did not improve the healing result above and beyond that obtained after mechanical debridement alone or in combination with saline irrigation. Hence, the study failed to demonstrate that professionally performed periodic subgingival irrigation with chlorhexidine or hydrogen peroxide, used alone or in combination with thorough mechanical debridement, has a significant therapeutic effect.