After a course of amoxycillin, isolates of Haemophilus species (both pathogenic and commensal) in the throats of children treated in general practice for acute respiratory tract infections are more antibiotic resistant for several weeks. In Oxfordshire (BMJ 2007;335:429–31) 119 children were studied; 71 were treated with a β lactam antibiotic (amoxicillin 70, cephradine 1) and 48 received no antibiotic. Haemophilus species was isolated from 85% at baseline, 92% at 2 weeks, and 97% at 12 weeks with little difference between the two groups. The mean minimum inhibitory concentrations (MICs) for ampicillin among Haemophilus species isolates at these times were 4.1, 2.7 and 2.7 μg/ml in the no-antibiotic group and 2.4, 9.2 and 5.7 μg/ml in the antibiotic group. The β lactamase encoding resistance element, ICEHin 1056, was recovered at the same times from 38%, 36% and 37% (no antibiotic) and from 32% 67% and 36% (antibiotic). Thus, prescribing an antibiotic doubled the likelihood of recovering this resistance element and more than tripled the mean MIC for ampicillin among isolates at the 2-week visit. The resistance element was recovered from 35% of children with Haemophilus isolates at the initial visit and from 83% of such children at some time in the study irrespective of treatment group. It is concluded that, while the increase in antibiotic resistance among …