Post-tonsillectomy bacteremia is a well-recognized aetiological factor in streptococcal endocarditis, and prophylactic penicillin has been recommended to reduce its incidence in susceptible patients undergoing tonsillectomy. Recent studies have shown a change in the microflora and an increase in the number of penicillin-resistant organisms in the tonsils of patients undergoing tonsillectomy. The aim of this study was to assess the incidence of post-tonsillectomy bacteraemia, to identify the micro-organisms associated with it and to review the suitability of penicillin in prophylactic regimens. The relationship between positive blood cultures and several clinical parameters such as fever, vomiting, pharyngeal discomfort, or dysphagia was also analysed. Of the 102 patients included in the study, 41 (40.1%) had positive post-tonsillectomy blood cultures. Haemophilus influenzae were isolated from 23 (56%) of the positive cultures and Streptococcus viridans in 15 (36.5%). Twenty-five per cent of H. influenzae produced beta-lactamase and only 30% of streptococci of the viridans group were penicillin-sensitive. Positivity of the blood cultures was not related to fever, discomfort, surgical technique, type of tonsil, or any of the parameters studied. Bacteraemia seems to be related to traction of the tonsil before dissection rather than direct spread of bacteria into the opened vessels. Using a beta-lactamase stable antibiotic instead of penicillin for prophylaxis would be more appropriate.