The aim of this study was to determine the extent of unnecessary investigation performed as part of the preoperative preparation of elective surgical patients in a teaching hospital and to audit the effect of guidelines and education designed to reduce unnecessary investigation. Guidelines were developed for preoperative anaesthetic investigation for elective surgical procedures in patients over one year of age according to internationally accepted criteria, with some adjustment for local differences in patient morbidity. Forrms outlining these criteria were placed in all operating theatres and anaesthetists were asked to determine whether tests performed were indicated or not, according to these criteria, over a two-week period, in each patient undergoing elective surgery. Tests indicated for surgical reasons were excluded. These same guidelines were then issued to all surgical departments along with explanatory lectures. The audit was repeated six months later and results compared. The incidence of over-investigation decreased from 13.8% to 11.6% (P = 0.03) without a significant increase in under-investigation (0.7 v 1.0%; P = 0.2). This study highlights the incidence of unwarranted screening tests in patients presenting for elective surgery and the role of protocols and ongoing education in reducing this incidence.