A 44-year-old man infected with human immunodeficiency virus had Hodgkin's disease, mixed cellularity, and malignant non-Hodgkin's lymphoma, diffuse large cell type. Colorimetric in-situ hybridization showed the Epstein-Barr virus (EBV) genome in the cells of the large cell non-Hodgkin's lymphoma and in the Reed-Sternberg cells and reactive lymphocytes of the Hodgkin's lymphoma. These results suggest that EBV may play a similar causative role in both neoplasms. This colorimetric method of hybridization, yielding results within 8 hours, is applicable to archival material and will be useful in further epidemiologic work associating EBV and lymphoid proliferations and malignancies.