OBJECTIVE: To determine whether remote, retroactive intercessory prayer, said for a group of patients with a bloodstream infection, has an effect on outcomes. DESIGN: Double blind, parallel group, randomised controlled trial of a retroactive intervention. SETTING: University hospital. SUBJECTS: All 3393 adult patients whose bloodstream infection was detected at the hospital in 1990-6. INTERVENTION: In July 2000 patients were randomised to a control group and an intervention group. A remote, retroactive intercessory prayer was said for the well being and full recovery of the intervention group. MAIN OUTCOME MEASURES: Mortality in hospital, length of stay in hospital, and duration of fever. RESULTS: Mortality was 28.1% (475/1691) in the intervention group and 30.2% (514/1702) in the control group (P for difference=0.4). Length of stay in hospital and duration of fever were significantly shorter in the intervention group than in the control group (P=0.01 and P=0.04, respectively). CONCLUSION: Remote, retroactive intercessory prayer said for a group is associated with a shorter stay in hospital and shorter duration of fever in patients with a bloodstream infection and should be considered for use in clinical practice.