The present study investigates stop signals and their effects on the pain and distress of patients undergoing a stressful medical procedure. Thirty-six chronic pain patients (17 men, 19 women) attending an out-patient operating theatre for diagnostic nerve blocks/local anaesthetic injections were allocated to one of two conditions (experimental and control). All patients received a standard information leaflet concerning the forthcoming injections. Additional information was given to those in the experimental group on four occasions (three orally, one written) before the injections which stated that they could halt the procedure at any time by saying 'stop'. Subjective measures of anxiety, pain, distress, sense of control over the procedure as well as observer ratings of patient distress and pain behaviour were obtained before, during and after the injections. After initial differences in pre-injection pain were controlled for the experimental group, patients rated themselves as less distressed during the injections and recorded lower state anxiety following treatment. In view of various methodological limitations of the present study its findings may only be accorded 'pilot study' status. These limitations are explored in the discussion along with their implications for a more robust replication study. Nevertheless the present findings provide tentative support for the hypothesis that the use of stop signals can reduce the stressful nature of diagnostic nerve blocks.