Heart-rate variability (HRV) analyzed by detrended fluctuation analysis (DFA), which resulted in a short-term fractal exponent - alpha, has prognostic implication in chronic phase of heart failure and survivors of myocardial infarction. We adopted DFA to study the acute-phase HRV in acute coronary syndromes (ACS) by recruiting 30 age- and co-morbidity-matched adults in acute-stress simulation in comparison with 33 consecutive ACS patients. The 30 volunteers got stepwise elevated alpha with increased intensity of exercise (0.95+/-0.050 to 1.07+/-0.084 to 1.20+/-0.083, p<0.05). The alpha of 33 ACS patients correlated with the complexity of post-MI course (1.004+/-0.0080 in non-complicated vs. 1.216+/-0.058 in complicated p<0.05). No significance existed in categories of age, ACS type, numbers of diseased vessel, infarct-related artery or Killip classification. The increased alpha value of DFA may imply unresolved cardiac stress which demands further attention.