Objectives: 1) To measure the incidence and impact of missed radiology and microbiology test results in an emergency department with an electronic test order and results viewing system, and 2) to assess the average times from test order to test availability. Methods: The study was conducted in the emergency department (ED) of a 370-bed metropolitan teaching hospital in Sydney, Australia. A computerised provider order entry (CPOE) system was used to order all diagnostic tests and view all test results. For microbiology and radiology tests electronic results were then printed for ED patients not admitted to the hospital to allow ED physicians to document follow-up. All radiology (n = 197) and microbiology (n = 66) tests ordered and results received for discharged ED patients were collected for a seven-day period. We measured the: 1) proportion of radiology and microbiology test results without follow-up for discharged patients; 2) impact of non follow-up on patient outcomes; 3) average time from radiological examination and microbiology specimen collection to reporting of results; and 4) average time from reporting of results to follow-up. Results: Two radiology (1.0%) and two microbiology reports (3.0%), all of which had negative findings, were never followed-up. Review of these patients' medical records indicated there was no impact on patient outcomes or management. The average time from radiological examination to reporting of a result was 1.5 days, and from microbiology specimen collection to reporting was 2.5 days. Eighty-nine percent of radiology and 68% of microbiology results were followed-up on the same day that they were available to physicians. Conclusions: Our rates of missed test results are lower than those reported from studies where paper ordering and reporting systems were used. This suggests that the availability of CPOE systems may reduce the risk of these events. Electronic result delivery, with electronic endorsement to allow documentation of follow-up of test results, may provide additional efficiency benefits and further reduce the risk of test results which are not followed up.