We performed a retrospective analysis of patients with adult-onset chronic active Epstein-Barr virus infection (CAEBV). First, we analyzed five patients (aged 28-72) diagnosed at our hospitals with EBV-infected clonally proliferating T cells. Four patients were administered cyclophosphamide/doxorubicin/vincristine/prednisone (CHOP) chemotherapy, but no remarkable decrease of viral load was observed in three of the patients. The other patient died 19 days after initiation of CHOP treatment due to disease progression. Addition of high-dose cytarabine to the regimens of two of the patients was discontinued shortly after administration, due to the development of grade 4 pericardial effusion. Together, these regimens may be insufficient for treating adult-onset CAEBV. We next reviewed 23 adult-onset CAEBV patients, adding 18 previously reported patients to the five patients described in the present study. T cells were frequently infected (87%), whereas NK- and T-cell types are known to be almost equally prevalent in childhood-onset cases. The time duration from the onset of disease to initiation of treatment averaged 20 months. Reports showed that 12 patients died; seven patients died at an average of 8 months after initiation of treatment. Patients' disease courses seemed to be rapidly progressive and more aggressive than those of childhood-onset cases. More cases must be studied to clarify clinical features and establish an optimal treatment strategy.