Extracorporeal circulation severely impairs the host immune system. Also decreases in circulatory lymphocytes correlate with greater degrees of surgical damage. In this study, the pattern of recovery from lymphocytopenia after extracorporeal membrane oxygenation was retrospectively evaluated from a prognostic viewpoint. The results showed that the circulatory lymphocyte count of survivors returned to normal levels within 5 days after their being weaned from extracorporeal membrane oxygenation, while the circulatory lymphocyte count of nonsurvivors remained at low levels. Because it is easy to measure the number of peripheral lymphocytes, the changing pattern of the circulatory lymphocytes might be a simple and reliable prognostic factor after extracorporeal membrane oxygenation.