Objectives. Low heart rate variability (HRV) is associated with poor prognosis after acute coronary events in men. In women, the prognostic impact is not well documented. The objective of this study was to assess the long-term predictive power of HRV on mortality amongst middle-aged women with coronary heart disease (CHD). Design, Settings and Subjects. Consecutive women below 65 years hospitalized for an acute coronary syndrome during a 3-year period in Stockholm were examined for cardiovascular prognostic factors including HRV, and followed for a median of 9 years. An ambulatory 24-h electrocardiograph was recorded during normal activities, 3-6 months after hospitalization. SDNN index (mean of the standard deviations of all normal to normal intervals for all 5-min segments of the entire recording) and the following frequency domain parameters were assessed: total power, high-frequency (HF) power, low-frequency (LF) power, very-low frequency (VLF) power and LF/HF ratio. Using Cox proportional hazards regression, the hazard ratios (HR) for each 25% decrease of the HRV parameters were assessed. Results. After controlling for the independent, significant predictors of mortality amongst the clinical variables, the following HRV parameters were found to be significant predictors of all-cause mortality: SDNN index [HR 1.56, 95% confidence intervals (CI) 1.19-2.05], total power (HR 1.21, 95% CI 1.08-1.35), VLF power (HR 1.22, 95% CI 1.09-1.36), LF power (HR 1.18 95%, CI 1.07-1.30) and HF power (HR 1.18, 95% CI 1.05-1.33). The results were essentially the same when cardiovascular mortality was used as end-points. The HRV parameters were stronger predictors of mortality in the first 5 years following the index event. Conclusion. Low HRV is a predictor of long-term mortality amongst middle-aged women with CHD when measured 3-6 months after hospitalization for an acute coronary syndrome, even after controlling for established clinical prognostic markers.