Physiological changes during the menstrual cycle in women with primary dysmenorrhea may affect their respiratory system, so this study aimed to examine various respiratory parameters in three phases of the menstrual cycle in these women. Materials and Methods:This study was conducted on 15 women with dysmenorrhea aged 18-25 years. The respiratory parameters were measured using a capnograph during the menstrual phase (MP; on days 3 or 4), follicular phase (FP; on days 10 or 11), and luteal phase (LP; on days 20 or 21). Results:The results of this study showed a significant difference in end-tidal CO 2 pressure in the follicular and luteal phase with that in the menstrual phase (P<0.01). Also, the respiration rate and oxygen saturation showed a significant difference in different phases of menstruation (P<0.01). Conclusion:The results of this study indicated that respiratory indices differ in various phases of the menstrual cycle in women with primary dysmenorrhea, which seems to be due to pain during the menstrual cycle and increased progesterone levels in the luteal phase.