Background: The possible role of heme oxygenase and its byproduct carbon monoxide (CO) in the regulation of blood pressure is under investigation. The aim of this study was to compare end tidal breath CO (ETCO) levels in women with gestation hypertension (GH) or pre-eclampsia to the levels in healthy pregnant and nonpregnant women. Materials and Methods: We prospectively performed ETCO measurements corrected for ambient CO (ETCOc) in two medical centers (Stanford, CA and Cleveland, OH). A Natus® CO-Stat® End Tidal Breath Analyzer (Natus Medical Inc., San Carlos, CA) was used. The study group included a convenience sample of 31 women with GH/pre-eclampsia (PE). Control groups included 46 nonpregnant healthy women, 44 first-trimester and 48 third-trimester pregnant healthy women. Results: Mean±SD ETCOc measurements were significantly lower in the GH/PE group compared to first-trimester (p = 0.004) and third-trimester (p = 0.001) normotensive pregnant and nonpregnant women. (p = 0.002) (1.36±0.30 vs 1.76±0.47, 1.72±0.42 and 1.78±0.54 ppm, respectively). The ETCOc values were ≤ 1.6 ppm in 89% of GH/PE women compared with, respectively, only 45, 54, and 46% of nonpregnant, first- and third-trimester normotensive pregnant women (p<0.05). ETCO measurements were not influenced by maternal age, parity, ethnicity, body mass index, gestational age or presence of household smokers. In the two centers, the controls had a similar mean ETCOc and the differences found remained significant when results for each center were anlyzed separately. Conclusions: ETCOc levels were found to be significantly lower in women with GH/PE. Futher investigation is required to determine if the lower CO levels reflect a deficient compensatory response to the increase in blood pressure or whether these are primary changes of significance to our understanding of the pathogenesis of GH/PE.