Objectives. To compare the analgesic effect of metoprolol and morphine in patients with chest pain due to suspected or definite acute myocardial infarction after initial treatment with intravenous metoprolol. Design. All patients, regardless of age, admitted to the coronary care unit at Uddevalla Central Hospital due to suspected acute myocardial infarction were evaluated for inclusion in the MEMO study (metoprolol–morphine). The effects on chest pain and side-effects of the two treatments were followed during 24 h. Pain was assessed by a numerical rating scale. Results. A total of 265 patients were randomized in this prospective double-blind study and 59% developed a confirmed acute myocardial infarction. In both treatment groups, there were rapid reductions of pain intensity. However, in the patient group treated with morphine, there was a more pronounced pain relief during the first 80 min after start of double-blind treatment. The side-effects were few and were those expected from each therapeutic regimen. During the first 24 h, nausea requiring anti-emetics was more common in the morphine-treated patients. Conclusion. In suspected acute myocardial infarction, if chest pain persists after intravenous beta-adrenergic blockade treatment, standard doses of an opioid analgesic such as morphine will offer better pain relief than increased dosages of metoprolol.