The influence of the menstrual cycle phase on cardiorespiratory parameters has not been studied in obese subjects. The aim of the study was to investigate the effect of menstrual cycle phase on resting and exercise cardiorespiratory variables. We studied ten obese subjects and ten normal subjects aged 18-25 years with regular menstrual cycle. The cardiorespiratory parameters at rest and maximal exercise were measured during 8-9 days of follicular phase (FP) and 20-21 days of luteal phase (LP). In the obese individuals oxygen uptake (VO2), carbon dioxide output (VCO2), minute ventilation (VE), tidal volume (VT) and systolic blood pressure (SBP) measured at rest were higher (p < 0.05) in FP compared to LP. In the normal individuals there was a significant increase (p < 0.05) in the resting SBP in FP. The exercise VO2, VCO2, VE and VT were higher (p < 0.05) in FP in the obese individuals, whereas these parameters were greater (p < 0.05) in LP in the normal individuals. Furthermore, heart rate, respiratory quotient, respiratory rate, SBP and diastolic blood pressure did not show any significant phase-related changes at exercise either in the obese or normal subjects. We conclude that the phase of the menstrual cycle may affect VO2, VCO2, VE and VT at maximal exercise in the both groups. We also conclude that the cycle phase may influence VE, VT and SBP at rest in the obese subjects and SBP at rest in the normal subjects.