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). Conclusions 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.