In order to determine the incidence and pattern of angina as a premonitory symptom of acute myocardial infarction, 577 consecutive patients with acute myocardial infarction were questioned shortly after hospital admission about the presence and pattern of chest pain prior to onset of infarction, with particular emphasis on the month prior to infarction. Two hundred and seventy-six patients (48 per cent) had no angina before infarction (Group I), whereas 301 (52 per cent) did. One hundred and seventy-nine patients (31 per cent) had a history of chronic angina, and of these, 75 had no change in the pattern of angina prior to infarction (Group II) while 104 noticed worsening of their symptoms in the month prior to infarction (Group III). One hundred and twenty-two patients (21 per cent) had new onset angina in the month prior to infarction (Group IV). The number of patients with unstable angina prior to infarction (Groups III and IV) was therefore 226 or 39 per cent of the total series. In patients with unstable angina, the increase in severity of symptoms or the development of new onset angina occurred within a period of 1 week or less in 69 per cent. Patients with a history of previous infarction or chronic angina had a higher incidence of unstable angina prior to infarction than patients without such a history (p less than 0.05). Patients with prior angina (Groups II, III, and IV) had a higher incidence of subendocardial infarction than patients without angina (p less than 0.05). The hospital mortality rate in the four groups did not differ significantly.