Aims: The aim of the present study is to investigate the impact of unwitnessed STEMI on prehospital delay and delay related cognitive and behavioural responses of the patients-Methods: Data were from the Multicentre MEDEA study with a total of 619 ST-elevated myocardial infarction (STEMI) patients. Data on sociodemographic, clinical and behavioural characteristics were collected at bedside. To assess the context of the symptom onset (witnessed or not) and the subsequent responses (behavioural, cognitive, and emotional), the Response to Symptoms Questionnaire was used. Associations with pre-hospital delay were computed using logistic regressions. Results: Among 619 STEMI patients, a total of 221 (36%) patients were alone during the acute phase of myocardial infarction, and 182 (29%) patients were living alone. These patients experienced severer fear during the acute phase compared with patients with witnessed STEMI but without perceiving their symptoms more seriously. The patients with unwitnessed STEMI were less likely to get into contact with lay others (e.g. family, friends or colleagues) (p<0.0001). In case they did, lay others were less likely to suggest the patients to seek medical help (p<0.0001). Unwitnessed STEMI was not associated with delay time. (196 vs. 213 mins, p= 0.3). However, there was a significant interaction between unwitnessed STEMI and living alone (p=0.02); the patients who experienced an unwitnessed STEMI and were living alone delayed the longest whilst patients who experienced a witnessed STEMI and were living alone delayed the shortest (250 vs. 161 mins, p=0.06). The patients who experienced an unwitnessed STEMI and were living alone had increased risk of delay longer than two hours compared with the patients with a witnessed STEMI who were living with someone (OR 1.7, 95% CI: 1.04-2.79) and the patients with unwitnessed STEMI who live alone(OR 2.54, 95% CI: 1.27-5.09). The prolonging effect of unwitnessed STEMI combined with living alone on prehospital delay remains significant only when the lay others did not suggest the patients to seek medical help (OR 2.22, 95% CI: 1.11-4.46). Conclusion: Unwitnessed STEMI combined with living alone was associated with higher risk of delay. However, this effect was moderated by lay others' behavioral response.