Clonal disease of large granular lymphocytes (LGLs) may arise from either CD3+ LGLs (LGL leukemia) or CD3- LGLs (natural killer [NK] cell leukemia). Other patients have chronic LGL proliferations that cannot be proven to be clonal (lymphoproliferative disease of granular lymphocytes [LDGL]). It was recently shown that clonally expanded CD3- LGLs from Japanese patients contain Epstein-Barr virus (EBV) DNA sequences, arguing for a direct causative role for EBV in NK cell leukemia. The aggressive clinical course and other clinical features of these Japanese patients differ markedly from the clinical features of LGL leukemia and CD3- LDGL patients in the United States and Europe, suggesting different pathogenic mechanisms. Therefore, we performed serologic and DNA hybridization studies for EBV in 31 patients from the United States and Europe (18 with LGL leukemia and 13 with chronic CD3- LDGL). All patients had serologic evidence for past infection with EBV. We did not detect EBV DNA sequences in peripheral blood mononuclear cell DNA from any of these patients in Southern blot hybridization analyses. EBV DNA sequences were detected after polymerase chain reaction amplification of peripheral blood mononuclear cell DNA in only 2 of 18 LGL leukemia patients and 4 of 13 chronic CD3- LDGL patients. These results argue against a direct causative role for EBV infection in LGL leukemia or chronic CD3- LDGL occurring in the United States and Europe.