BACKGROUND: The National Institute for Health and Clinical Excellence (NICE) issued guidance on surgical techniques for tonsillectomy during a national audit of surgical practice and postoperative complications. OBJECTIVE: To assess the impact of the guidance on tonsillectomy practice and outcomes. DESIGN: An interrupted time-series analysis of routinely collected Hospital Episodes Statistics data, and an analysis of longitudinal trends in surgical technique using data from the National Prospective Tonsillectomy Audit. PARTICIPANTS: Patients undergoing tonsillectomy in English NHS hospitals between January 2002 and December 2004. MAIN OUTCOME MEASURE: Postoperative haemorrhage within 28 days. RESULTS: The rate of haemorrhage increased by 0.5% per year from 2002, reaching 6.4% when the guidance was published. After publication, the rate of haemorrhage fell immediately to 5.7% (difference 0.7%: 95% CI -1.3% to 0.0%) and the rate of increase appeared to have stopped. Data from the National Prospective Tonsillectomy Audit showed that the fall coincided with a shift in surgical techniques, which was consistent with the guidance. CONCLUSION: NICE guidance influenced surgical tonsillectomy technique and in turn produced an immediate fall in postoperative haemorrhage. The ongoing national audit and strong support from the surgical specialist association may have aided its implementation.