Among 88 renal transplant recipients evaluated for a change in Epstein-Barr virus (EBV) antibody status in the period after transplant, 22 showed a 4-fold rise and eight showed an 8-fold or greater rise in EBV antibody. Among the patients with an 8-fold or greater EBV ANTIBODY RISE, THE OCCURRENCE OF FEVER WAS FREQUENT, ONE PATIENT DEVELOPED A LYMPHOPROLIFERATIVE reaction, and one died with a malignant EBV infection. Patients without pretransplant antibody showed a longer mean time to antibody rise (104 +/- 23 days) than did those patients with pretransplant antibody (19 +/- 7 days). The longer incubation period in patients without pretransplant antibody was in the expected range for primary EBV infections. Both primary and secondary (reactivation) EBV infections occur in renal transplant patients. These infections may be assoicated with prolonged fever, and in unusual circumstances, may cause dramatic lymphoproliferative disease.