CMV disease is a challenge to the success of renal transplantation. Recently, the investigators analyzed data from 792 renal transplant recipients performed at our hospital between 1999 and 2005. After the usual exclusions, 663 patients were analyzed. This population showed that the incidence of CMV disease is stable and occurs in approximately 20-22% of all patients and invasive disease in approximately 5% every year. In seronegative patients and those receiving anti-lymphocyte (AL), CMV prophylaxis, done with ganciclovir for 90 days is our routine and in the majority of transplant centers. In seropositive patients without associated risk factors (such as the use of AL) universal prophylaxis is not done. Rather, in this group, early diagnosis, by detection of antigenemia or viremia by quantitative PCR, is performed in patients who show symptoms compatible with CMV disease. In the investigators analysis the incidence of CMV disease in seropositive patients is around 16%. These patients are usually hospitalized and treated with GCV IV for 14-21 days. This leads to an additional costs of admissions, biopsies for the diagnosis of disease invasion, etc. Besides these costs, the survival of the grafts in the long run is lower in patients with CMV disease than in those without CMV, particularly when associated with acute rejection. In recent years, monitoring of viremia (PCR / antigenemia) and preemptive treatment when it reaches substantial values, have increasingly been suggested. Patients in whom the detection of viremia in progressive values is detected would be treated as outpatients before the disease develops. To turn this hypothesis into reality, there is an urgent need to define cutoff values for CMV-PCR in the detection of developing CMV disease.