Current thoracic artificial lungs (TALs) have blood flow impedances greater than the natural lungs, which can result in abnormal pulmonary hemodynamics. This study investigated the impedance and gas transfer performance of a compliant TAL (cTAL). Fluid-structure interaction analysis was performed using ADINA (ADINA R&D Inc., Watertown, MA) to examine the effect of the inlet and outlet expansion angle, θ, on device impedance and blood flow patterns. Based on the results, the θ = 45° model was chosen for prototyping and in vitro testing. Glycerol was pumped through this cTAL at 2, 4, and 6 L/min at 80 and 100 beats/min, and the zeroth and first harmonic impedance moduli, Z(0) and Z(1), were calculated. Gas transfer testing was conducted at blood flow rates of 3, 5, and 7 L/min. Fluid-structure interaction results indicated that the 45° model had an ideal combination of low impedance and even blood flow patterns and was thus chosen for prototyping. In vitro, Z(0) = 0.53 ± 0.06 mm Hg/(L/min) and Z(1) = 0.86 ± 0.08 mm Hg/(L/min) at 4 L/min and 100 beats/min. Outlet PO(2) and SO(2) values were above 200 mm Hg and 99.5%, respectively, at each flow rate. Thus, the cTAL had lower impedance than hard shell TALs and excellent gas transfer.