BACKGROUND: Heart rate variability (HRV) is an efficient non-invasive method to represent the autonomic nervous system (ANS) activity. In clinical settings, HRV has been demonstrated to be a prognostic indicator related to cardiovascular diseases (CVD). A common approach for measuring HRV is to record within an isolated condition. However, HRV varies from day to day, hence an isolated measurement is often not suitable for reflecting a true change in ANS status. According to previous research utilizing isolated HRV recordings, both aerobic fitness and body mass index (BMI) are correlated with HRV. Yet, the extent to which aerobic fitness and BMI independently relate to HRV is less clear, especially when HRV is expressed as an average over time or as the coefficient of variation (CV) of multiple measures. Furthermore, our understanding of these relationships among young adult women subjects is limited as the majority of HRV research has involved men. PURPOSE: To determine if HRV collected over a period of days is associated with aerobic fitness and BMI in young adult women. METHODS: Healthy, untrained young adult women (n=30; 20.6±1.2 yr) who were overweight (26.9 ±7.1 kg/m2) participated in this study. HRV was measured for three consecutive days at the same time in the early morning and averaged (3dayM). A maximal graded exercise test on the treadmill was performed to evaluate the peak oxygen consumption (VO2peak) after 3-day HRV data collection. BMI was calculated using subjects’ height and weight. Independent associations between HRV (i.e., 3dayM and corresponding CV [3dayCV]) and aerobic fitness and BMI were evaluated using correlations. RESULTS: Aerobic fitness (VO2peak) was not correlated with any of the HRV parameters for the 3dayM and 3dayCV values (all P>0.05). BMI was not correlated with any of the HRV parameters for the 3dayM values (all P>0.05), however, BMI was correlated with all but one HRV parameter (HFCV, p>0.05) for 3dayCV: SDNNCV (r=0.471, p=0.009), RMSSDCV (r=0.396, p=0.030), LFCV (r=0.499, p=0.005), SD1CV (r=0.394, p=0.031), and SD2CV (r=0.426, p=0.019). CONCLUSION: The results of this study showed that HRVCV values were significantly correlated to BMI among 30 healthy, untrained women subjects over 3-day measurement.