BACKGROUND The thymus grows rapidly during fetal life and continues to grow during childhood. When a child undergoes a median sternotomy during cardiac surgery, most of the thymus is removed to gain access to the mediastinum. What happens to the residual thymic tissue in the long term? OBJECTIVE To test the hypothesis that residual thymic tissue left after sternotomy regenerates into an identifiable thymus and is visible on MRI. METHODS We retrospectively reviewed the cardiac MR images obtained over a 14-month period in all children under the age of 17 years who had undergone a median sternotomy (n=62) to establish the presence/absence of a thymus. We also reviewed the cardiac MR images obtained over the same time period in children who had never undergone open cardiac surgery (n=37). RESULTS In the sternotomy group, 18 patients (29%) had an identifiable thymus on MR images, compared to 92% (n=34) in the nonsternotomy group. This difference was statistically significant. CONCLUSIONS The majority of children in the study group did not have a visible thymus on MR images, which suggests that in these children any residual thymic tissue left postoperatively does not regenerate.