Background: Failure to follow-up test results is a critical safety issue. The objective was to systematically review evidence quantifying the extent of failure to follow-up test results and the impact on patient outcomes. Methods: The authors searched Medline, CINAHL, Embase, Inspec and the Cochrane Database from 1990 to March 2010 for English-language articles which quantified the proportion of diagnostic tests not followed up for hospital patients. Four reviewers independently reviewed titles, abstracts and articles for inclusion. Results: Twelve studies met the inclusion criteria and demonstrated a wide variation in the extent of the problem and the impact on patient outcomes. A lack of follow-up of test results for inpatients ranged from 20.04% to 61.6% and for patients treated in the emergency department ranged from 1.0% to 75% when calculated as a proportion of tests. Two areas where problems were particularly evident were: critical test results and results for patients moving across healthcare settings. Systems used to manage follow-up of test results were varied and included paper-based, electronic and hybrid paper-and-electronic systems. Evidence of the effectiveness of electronic test management systems was limited. Conclusions: Failure to follow up test results for hospital patients is a substantial problem. Evidence of the negative impacts for patients when important results are not actioned, matched with advances in the functionality of clinical information systems, presents a convincing case for the need to explore solutions. These should include interventions such as on-line endorsement of results.