Objective: The purpose of this study was to determine the validity of the SenseWear Armband (SWA) in assessing the energy expenditure at different exercise intensities during midfollicular and luteal phases of the menstrual cycle. Material and Methods: Twenty volunteer women athletes (age: 20.6±1.6 yrs, height: 168.3±7.3 cm; menstrual cycle: 28.8±2.1 days) with regular menstrual cycle participated in the study. Participants performed treadmill running exercise for 10 min at 35% and 75% of their VO2max in their midfollicular (days 7-9) and luteal phases (days 21-23). Phases of the menstrual cycle were confirmed by hormone analysis. Energy expenditure during the running exercise was assessed simultaneously by the indirect calorimetric method, and the SWA. Multi-way ANOVA was used for the effect of method (2) x phase (2) x intensity (2) on energy expenditure. In addition, the Pearson correlation coefficient was determined between the variables. Results: Estradiol and progesterone levels measured in the midfollicular phase were significantly higher than in luteal phase (p<0.05). Energy expenditure at 75% intensity was significantly higher than energy expenditure at 35% intensity (p<0.05). No significant method (p>0.05) and phase (p>0.05) effect was found in energy expenditure. There was no significant method x phase, phase x intensity, and method x phase x intensity interaction (p>0.05), however method x intensity interaction was significant (p<0.05). At the midfollicular phase the correlations between energy levels from indirect calorimetric and SWA methods were not significant (for 35%, r=0.382; for 75%, r=0.147; p>0.05). In contrast, significant correlation was found between energy levels from the indirect calorimetric, and the SWA methods at the luteal phase (for 35%, r=0.495; for 75%, r=0.748; p<0.05). Conclusions: The results of the present study indicated that energy expenditures at different menstrual phases were similar and that SWA was a valid method in assessing energy expenditure at different exercise intensities during the menstrual cycle.