The aetiology of post-tonsillectomy fever is obscure. Bacteraemia during surgery, anaesthetic agents and the inflammatory response of tissue to injury have been implicated. A prospective study was undertaken in 100 consecutive children to evaluate the occurrence and severity of fever in the 24 h after tonsillectomy and its relationship to bacteraemia during surgery and qualitative and quantitative cultures (colony counts) of organisms in tonsil core tissue. Fifty-four patients had a fever (> 37.5 degrees C) postoperatively, of whom, 30 had a fever greater than 38 degrees C. Blood cultures during tonsillectomy were positive in 22 patients. There was no statistically significant difference between the occurrence of fever and the techniques of tonsillectomy and haemostasis. There was also no association between positive blood, core or surface cultures and the incidence or severity of fever nor any association between colony count in core cultures and fever. Our results suggest that postoperative fever in the 24 h following tonsillectomy is not caused by infection.