Previous reviews of electronic decision-support systems (EDSS) have often treated them as a single category, and factors that may modify their effectiveness of EDSS have not been examined. The objective was to summarise the evidence associating the use of computerised decision support and improved patient outcomes. PubMed/Medline and the Database of Abstracts were searched for randomised controlled trials (RCT) of EDSS from 1 January 1994 to 31 January 2006. Twenty-four RCT studies from 97 reviewed were selected, eight of them examined systems supporting decisions for patients who presented with an acute illness, and 16 studies enrolled patients with chronic conditions. Overall, 13 (54%) of the studies showed a positive result, and 11 (46%) were with no impact. Critiquing and consultative systems showed the impact in 71% and 47% of studies, respectively. All systems targeting decisions related to acute disease improved patient outcomes compared with 38% of systems focused on the management of chronic conditions (P = 0.005). Provision of EDSS improves prescribing practices and treatment outcomes of patients with acute illnesses; however, EDSS were less effective in primary care. Complex interventions as clinical EDSS may require new metrics of assessment to describe the impact on patient outcomes.