Bilateral lung transplantation without cardiopulmonary bypass consists of two sequential single lung transplantations. Variations in ventilatory status during the procedure led us to study the (PaCO2-PE'CO2) gradient to see if PE'CO2 might reflect PaCO2. The gradient was studied in 14 patients at six times during operation. (PaCO2-PE'CO2) (kPa) was mean 1.97 (SD 0.7) after induction, 3.2 (1.4) during single lung ventilation, 1.9 (1.1) after clamping of the contralateral pulmonary artery, 2.96 (1.6) after ventilation and vascularization of the first transplant and the remaining native lung, 0.99 (0.8) during single lung ventilation with the first transplant and 1.3 (0.8) during ventilation of both transplants. With ventilation by the allograft lung(s) alone, the small (PaCO2-PE'CO2) value demonstrated improvement in ventilatory status, enabled PaCO2 to be assessed by PE'CO2 and demonstrated efficiency of the grafts.