An oligonucleotide probe directed against small RNAs encoded by EBV gene was used to detect EBV genome by in situ hybridization combined with immunophenotype analysis. The results showed that (1) in 18 of 19 cases (94.7%) atypical lymphoid cells (ALC) expressed T-cell differentiation antigen, 15 cases were CD3 positive, 9 cases were UCHL1 positive, 6 cases were both CD3 and UCHL1 positive, no ALC gave positive reaction to B-cell and histiocyte markers. (2) 15 of 19 cases gave positive reaction to EBER (small RNAs encoded by EBV gene) by in situ hybridization analysis and ALC were reactive to T-cell markers in all 15 cases but one. The results of this study suggest that EBV infection is also present in midline malignant reticulosis, which is characterized by active T-cell proliferation. The role and effect of EBV infection in pathogenesis of lymphoproliferative disorders was discussed.