To assess the periodontal clinical response and the possible adverse effects of the clarithromycin combined to periodontal mechanical therapy in the treatment of patients with generalized aggressive periodontitis.Forty patients were select and randomly assigned into two groups: Group clarithromycin with 20 subjects that received one-stage full-mouth ultrasonic debridement (FMUD) associated with clarithromycin (500 mg - 12/12 hours) for 3 days; group placebo with 20 subjects that received FMUD associated with placebo. Probing depth (PD), gain in clinical attachment level (CAL) and bleeding on probing (BOP) were evaluated at baseline, 3 and 6 months post- operatively.