The purpose of the present study was to evaluate the effects of tetracycline fibres (TCF) as an adjunct to scaling in the treatment of generalised aggressive periodontitis and to compare the effects with mechanical treatment only. Ten patients, 24-39 years old referred for treatment to the Department of Preventive Dentistry, Periodontology and Implant Biology, Aristotle University of Thessaloniki took part in the study. A split-mouth experimental design was used. Measurements referring to bleeding on probing (BOP), pocket depth (PD) and clinical attachment level (CAL) were performed at 12 sites randomly selected. Clinical recordings were made at baseline, 2 and 6 months, after treatment. Subgingival plaque samples were taken for microbiological analysis using the 'checkerboard' DNA-DNA hybridisation technique at baseline, immediately after treatment and at 2 and 6 months. Full-mouth scaling and root planing were performed, with the exception of 2 pre-selected sites, which served as controls. Tetracycline fibres were applied in 5 pockets located in the same half mouth. Analysis of clinical findings showed that mechanical instrumentation in combination with TCF application led to a greater improvement in clinical parameters than scaling and root planing only. Microbial analysis showed a statistically significant greater reduction in the percentages of detection for B. forsythus, P. nigrescens and A. naeslundii genospecies II in pockets where tetracycline fibres were applied. In conclusion, the clinical and microbiological data of the present study suggest that the adjunctive use of TCF improves the clinical response of scaling and root planing in aggressive periodontitis patients.