We conducted three prospective studies of Haemophilus influenzae in different groups of children. Pharyngeal swab samples were taken (i) from 1382 healthy infants and children between 0 and 10 years of age (group 1), attending well child clinics (n = 438), day care centres (n = 440) and elementary schools (n = 504), and (ii) from 322 children aged 2-10 years (group 2), clinically diagnosed as having upper respiratory tract infection. Pharyngeal swab samples and sinus aspirates were obtained from 49 children between 2 and 9 years of age (group 3), clinically diagnosed as having sinusitis. H. influenzae was isolated in similar rates from 315 (22.7%) of children in group 1, 72 (22.3%) of children in group 2 and 12 (24.4%) of children in group 3. Serotype b comprised 7, 5.2 and 2% of all H. influenzae isolates for group 1, 2 and 3, respectively. Production of beta-lactamase was detected in 1.0% of H. influenzae type b isolates in group 1, 1.2 and 6.1% of all isolates in group 2 and 3, respectively. There were no beta-lactamase negative ampicillin-resistant strains. (C) 2003 Elsevier B.V. and the International Society of Chemotherapy. All rights reserved.