Background: Electrical impedance tomography (EIT) is a noninvasive imaging technique using impedance to visualize and measure blood volume changes. Study objective: To examine the validity of EIT in the measurement of hypoxic pulmonary vasoconstriction (HPV) and hyperoxic pulmonary vasodilation in healthy volunteers and COPD patients. Participants: Group 1 consisted of seven healthy volunteers (mean age, 46 years; age range, 36 to 53 years). Group 2 comprised six clinically stable COPD patients (mean age, 65 years; age range, 50 to 74 years). Interventions: EIT measurements were performed in healthy subjects while they were breathing room air, 14% oxygen (ie, hypoxia), and 100% oxygen (ie, hyperoxia) through a mouthpiece. Maximal impedance change during systole (ΔZsys) was used as a measure of pulmonary perfusion-related impedance changes. Stroke volume (SV) was measured by means of MRI. In the COPD group, EIT and SV also were determined, but only in room air and under hyperoxic conditions. Results: The data were statistically compared to data for the room air baseline condition. In the volunteers, the mean (± SD) ΔZsys for the group was 352 ± 53 arbitrary traits (AU) while breathing room air, 309 ± 75 AU in hypoxia (p < 0.05), and 341 ± 69 AU in hyperoxia (not significant [NS]). The mean MRI-measured SV was 83 ± 21 mL while breathing room air, 90 ± 29) mL in hypoxia (NS), and 94 ± 19 mL in hyperoxia (p < 0.05). In the COPD patients, the mean ΔZsys for this group was 222 ± 84 AU while breathing room air and 255 ± 83 AU in hyperoxia (p < 0.05). In this group, the SV was 59 ± 16 mL while breathing room air and 61 ± 13 mL in hyperoxia (NS). Thus, the volunteer EIT response to hypoxia is not caused by decreased SV, because SV did not show a significant decrease. Similarly, in COPD patients the EIT response to hyperoxia is not caused by increased SV, because SV showed only a minor change. Conclusion: EIT can detect blood volume changes due to HPV noninvasively in healthy subjects and hyperoxic vasodilation in COPD patients.