The systemic distribution of Epstein-Barr virus (EBV) genomes was studied in paraffin-embedded tissues from 12 fatal cases of Post-transplantation lymphoproliferative disease (PTLD), using an in situ hybridization technique employing an alpha-35S-dCTP-radiolabeled BamHI-W fragment of EBV DNA. The presence of EBV was documented in various PTLD-involved organs. The hybridization signal for the virus localized predominantly in the abnormal lymphoid cells, but signals also were detected in hepatocytes and/or adrenal cortical cells in five cases. The distribution of autoradiographic label within the lymphoid cells was focal and its intensity varied from field to field suggesting a nonuniformity of the viral genomic load in the infected tissues. Recruitment of EBV genome-bearing cells was not observed into inflammatory mononuclear infiltrates found in organs without histopathologic evidence of PTLD.