Primary effusion lymphoma (PEL) is a rare type of non-Hodgkin lymphoma that usually develops in immunosuppressed patients infected with human herpes virus-8 (HHV-8) in conjunction with human immunodeficiency virus (HIV) infection. However, there are several reports of HHV-8-related HIV-negative cases and HHV-8-unrelated HIV-negative cases, mainly in immunodeficient and elderly patients. Here, we report one case of HHV-8-related HIV-negative PEL with gastric cancer (case 1) and one case of HHV-8-unrelated HIV-negative effusion-based lymphoma (case 2), both in elderly men. A 73-year-old man and a 79-year-old man were admitted because of lymphomatous effusions, and no mass was detectable in both cases. They were diagnosed as having malignant effusion lymphoma on the basis of cytological findings indicating atypical lymphoid cells and the expression of CD20 and CD79a. To detect evidence of HHV-8 infection in neoplastic cells, immunocytochemical staining for ORF73/latent nuclear antigen-1 (LNA-1) was performed. The results revealed that case 1 was ORF73-positive, and case 2 was ORF73-negative. Rituximab-based chemotherapy (R-THPCOP: rituximab, pirarubicin, cyclophosphamide, vincristine, prednisolone) was administered to both patients and complete remission was achieved in both. Compared to most HIV-positive PEL cases, these two cases showed a good response to chemotherapy. In cases of PEL, we should focus on HHV-8 infection and HIV status for determining prognosis.