IgG antibody titres to heat shock protein 27 (anti-Hsp27) were measured to determine whether these titres were affected in patients admitted with acute coronary syndrome. Blood samples were taken from 94 patients admitted with acute coronary syndrome. Anti-Hsp27 IgG titres were determined using an in-house enzyme-linked immunosorbent assay (ELISA) in the first and second 12 h after the onset of symptoms and compared with values for 81 age- and sex-matched control subjects. Median antibody titres to Hsp27 in the first sample from patients whose diagnosis was a myocardial infarction (n = 42) was 0.41 absorbancy units (range 0.28–0.57) and for those with unstable angina (n = 52) was 0.31 (range 0.20–0.42), both being significantly higher than for controls (n = 81), which was 0.08 (range 0.05–0.15) (P<0.05). However, titres fell in the second samples collected in the coronary syndrome patients and were then no longer significantly different from controls (P>0.05). Myocardial infarction patients also had significantly higher anti-Hsp27 titres in the first 12 h than patients with unstable angina (P<0.05), but again the difference in the second sample did not reach statistical significance (P>0.05). Serum antibody titres to Hsp27 rise and fall rapidly after the onset of acute coronary syndrome, and may be an early marker of myocardial ischaemia as patients with myocardial infarction or unstable angina both had high titres.