Nerve injury associated with cardiac surgery and anesthesia is a well-recognized complication with a predilection for the upper half of the body. We report four cases of common peroneal nerve palsy following cardiac surgery that were not subject to external compression to fibular head. The diagnosis of nerve palsy was delayed because of a complicated postoperative course and intensive care, which prevented our determining the causes. The mechanisms of nerve injury are reviewed. We postulate that the etiology is multifactorial, including, old age, subnormal body habitus, prolonged knee flexion and rotation, coexisting disease, postoperative cardiovascular complications, and cardiopulmonary bypass.