The Viriato Study is a nationwide, prospective, multicenter surveillance study of the antimicrobial susceptibility of bacterial pathogens commonly associated with community-acquired respiratory tract infections in Portugal. In years 2001 and 2002, 31 laboratories participated in the study with a total of 2,600 strains, with testing undertaken in a central laboratory. Of the 486 Streptococcus pyogenes strains isolated from patients with acute tonsillitis all were susceptible to penicillin and other beta-lactams but 21.6% were resistant to erythromycin, clarithromycin and azithromycin. From patients with lower respiratory tract infection 1,071 strains of Streptococcus pneumoniae, 811 of Haemophilus influenzae, and 232 of Moraxella catarrhalis were studied. Among S. pneumoniae isolates 23% were resistant to penicillin (6.7% showing high-level resistance), 9.1% to cefuroxime, 1.0% to amoxycillin, 12.5% to erythromycin, clarithromycin and azithromycin, 10.9% to tetracycline, 29.8% to co-trimoxazol, and 3.0% to ofloxacin. Beta-lactamase was produced by 13.1% of H. influenzae and 94.4% of M. catarrhalis. In H. influenzae resistance to clarithromycin was 12.5% and to co-trimoxazole 15%. Most strains were susceptible to amoxycillin/clavulanate, cefuroxime, azithromycin, tetracycline and ciprofloxacin. In M. catarrhalis resistance to co-trimoxazole was 25%. All strains were susceptible to amoxycillin/clavulanate, cefuroxime, clarithromycin, azithromycin, tetracycline and ciprofloxacin. As verified in previous years of the Viriato Study, penicillin was the most active antimicrobial against S. pyogenes and amoxycillin/clavulanate the most active in vitro simultaneously against H. influenzae, S. pneumoniae and M. catarrhalis isolated from patients with community-acquired lower respiratory tract infection in Portugal.