Background. There has been ethical concern that participants in psychiatric research will become distressed and their mental state might worsen.Method. A systematic search was carried out for studies that examined distress following participation in research that involved the assessment of psychiatric state or associated risk factors. There were 46 relevant studies.Results. A minority of participants become distressed immediately after participation, with distress more likely in studies of traumatic experiences. There is limited evidence on longer-term effects, but what there is suggests no adverse impact. Positive reactions to participation show little association with distress and these are more common than negative reactions. Very few studies of distress in research have used control groups to establish causal associations. However, what evidence there is suggests no causal role, including for research on suicidality. Researchers in this area have made a range of suggestions about ethical practice.Conclusions. A minority of participants in psychiatric research become distressed, but there is no evidence of longer-term harm. Nevertheless, researchers need to take account of ethical concerns in designing studies. Future research in the area needs to be carried out with stronger designs involving control groups.