Abstract: A hollow fiber oxygenator includes a housing defining a blood chamber between an inner core wall and an outer wall. The chamber includes at least one blood inlet and at least one blood outlet. A plurality of hollow semi-permeable fibers extend through the chamber. Each fiber has an open inlet and and an open outlet end, the ends being sealed from the chamber by sealings. The oxygenator further includes a gas inlet on one end of the chamber in communication with the inlet ends of the fibers, and a gas outlet on the other end of the chamber in communication with the outlet ends of the fibers. Blood flows into the chamber and across the exterior surfaces of the fibers to effectuate gas transfer therebetween. A partitioning wall divides the chamber into at least two sections. In one of the sections, the blood flow is co-current with the gas flow, while in another section the blood flow is counter-current with respect to the gas flow.