Single center, randomized placebo controlled crossover clinical trial with a one-year follow-up. Eighty participants with Aggregatibacter actinomycetemcomitans-associated chronic or aggressive periodontitis were randomized into two treatment groups. A: Antibiotics (3/d 500 mg metronidazole plus 375 mg amoxicillin for 7 days) during the first, non-surgical phase of periodontal therapy (T1), and placebo during the second, surgical phase (T2). B: Placebo during T1, and antibiotics during T2. Persistence of sites with a probing depth (PD) >4 mm and bleeding on probing (BOP) was the primary outcome. Evaluations were made three months after T1, as well as 6 and 12 months after T2.