Cytomegalovirus (CMV) continues to be a clinical problem, impairing the overall success rate of transplantation, either through direct involvement of a variety of end-organs or by inducing indirect effects such as graft rejection. We review here how the virus manages to evade host immune responses and replicate extensively in allograft recipients. Recent studies show that the quantity of CMV (viral load) is related directly to the development of CMV disease. We review how clinically significant levels of CMV viral load can be defined and summarize the results of studies showing that a high CMV viral load is the major determinant of CMV disease, explaining the previously reported risk factors of pre-transplant serostatus and the post-transplant detection of CMV viremia.Note