ObjectiveRecent studies have shown that high resting end tidal CO2(PECO2) is a marker for blood pressure sensitivity to high sodium intake by normotensive humans. The present study investigated the association of restingPECO2with resting blood pressure in participants in the Baltimore Longitudinal Study on Aging (BLSA).DesignPECO2of 314 healthy participants (153 men and 161 women, aged 21–79 years) was measured by a respiratory gas monitor during 25 min of seated rest, and blood pressure every 5 min by an automated oscillometric system. The independent associations ofPECO2and other variables with systolic (SBP) and diastolic (DBP) blood pressure were analysed via multiple regression. Sex differences in age-associated changes inPECO2and in the role of pulmonary capacity in thePECO2-blood pressure relationships were also studied.ResultsRestingPECO2was an independent predictor of SBP in women (β=0.215;P< 0.0015; overallr2= 0.27;P< 0.0001), and accounted for more than 10% of the variance in SBP in women over age 50 years. No such associations ofPECO2with SBP of men, or with DBP of men or women, were observed.PECO2was lower in younger women (36.0 ± 0.9 mmHg) than in younger men (40.4 ± 2.0), but higher in older women (37.1 ± 0.5) than in older men (35.5 ± 0.4).ConclusionsPECO2is an independent determinant of resting SBP in women, especially those aged 50 years or more. The origin of individual differences in restingPECO2and its possible role in the development of chronic hypertension remain to be clarified.