We prospectively determined the role of commonly used antibiotics in the emergence of antimicrobial resistance among the predominant pathogens associated with the respiratory tract. Clinical isolates of Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis from otic exudates, exudates obtained by puncture of the paranasal sinuses, sputum samples or blood cultures obtained from hospital inpatients with symptoms of significant upper or lower respiratory tract infection were analyzed. Between 1983 and 1992 a statistically significant increase in strains of S. pneumoniae and H. influenzae resistant to ampicillin, erythromycin and co-trimoxazole was detected. A significant increase in strains of M. catarrhalis resistant to ampicillin was also found. The increase in the consumption of aminopenicillins, co-trimoxazole and macrolides was associated with an increase in strains of S. pneumoniae, H. influenzae and M. catarrhalis resistant to these agents.