Abstract Cytomegalovirus (CMV) infections in renal transplant recipients can affect the gastrointestinal tract, but significant clinical manifestations are seldom seen. We hypothesize that subclinical involvement of the gastrointestinal tract may be quite frequent during CMV infection. In order to study this, we measured intestinal permeability by calculating the urinary lactulose mannitol (LM) excretion ratio after oral administration of lactulose and mannitol (normal < 0.030) in patients with symptomatic and asymptomatic CMV infection. A total of 111 patients were enrolled in the study, 104 of whom were tested on postoperative day (POD) 10. Twenty-nine patients developed CMV infection, 12 of whom could be studied with the permeability test (median POD 40). Another nine patients without CMV infection were also studied at day 40 and served as controls. The LM ratio increased significantly during CMV infection compared to measurements before active infection (median 0.060 vs. 0.030, P< 0.01) and was significantly higher during the infection than in the control group (median 0.007, P< 0.01). No correlation could be found between the LM ratio and viral load, humoral response to the virus, or symptomatology of infection. We conclude that an increased intestinal permeability is found in a substantial number of patients with an active, albeit asymptomatic, CMV infection after renal transplantation. Pathophysiological mechanisms and clinical implications remain speculative but will be subject to further study.