The aim of this study was to evaluate the effect of the association of locally delivered doxycycline 10% and scaling and root planing in the subgingival plaque of smokers. Sixteen smokers with chronic periodontitis and a minimum of four pockets (> or = 5 mm) on anterior teeth that bled on probing were selected. Patients were randomly assigned to one of the following groups: scaling and root planing (SRP) or scaling and root planing followed by local application of doxycycline (SRP-D). Subgingival plaque samples were collected from initially moderate (5 to 6 mm) and deep (> or = 7 mm) pockets at baseline and 3 months. Polymerase chain reaction (PCR) analysis was used to detect the frequency of Actinobacillus actinomycetemcomitans (Aa), Porphyromonas gingivalis (Pg), and Tannerella forsythensis (Tf). No statistically significant difference was found in the reduction of Aa in either the SRP-D or SRP group (P > 0.05). The reduction in Tf, Pg, and Tf + Pg was statistically significant for SRP-D only (P = 0.016, 0.027, and 0.027, respectively). The proportion of sites free of Tf at 3 months was 53% for SRP-D and 9% for SRP (P = 0.02). For Pg, this proportion was 82% and 40%, respectively (P = 0.05). The use of locally delivered doxycycline may promote the elimination of T. forsythensis and P. gingivalis in a greater proportion of sites compared to conventional scaling and root planing in smokers.751600-4