Objective: To assess the impact of a computerised pathology order entry system on laboratory turnaround times and test ordering within a teaching hospital. Methods: A controlled before and after study compared test assays ordered from 11 wards two months before (n = 97 851) and after (n = 113 762) the implementation of a computerised pathology order entry system (Cerner Millennium Powerchart). Comparisons were made of laboratory turnaround times, frequency of tests ordered and specimens taken, proportions of patients having tests, average number per patient, and percentage of gentamicin and vancomycin specimens labelled as random. Results: Intervention wards experienced an average decrease in turnaround of 15.5 minutes/test assay (range 73.8 to 58.3 minutes; p<0.001). Reductions were significant for prioritised and non-prioritised tests, and for those done within and outside business hours. There was no significant change in the average number of tests (p = 0.228), or specimens per patient (p = 0.324), and no change in turnaround time for the control ward (p = 0.218). Use of structured order screens enhanced data provided to laboratories. Removing three test assays from the liver function order set resulted in significantly fewer of these tests being done. Conclusions: Computerised order entry systems are an important element in achieving faster test results. These systems can influence test ordering patterns through structured order screens, manipulation of order sets, and analysis of real time data to assess the impact of such changes, not possible with paper based systems. The extent to which improvements translate into improved patient outcomes remains to be determined. A potentially limiting factor is clinicians' capacity to respond to, and make use of, faster test results.