Human herpesvirus type 8 (HHV-8; Kaposi's sarcoma-associated herpesvirus) is a recently isolated human herpesvirus frequently identified in Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. Here we report three cases of HHV-8-bearing solid lymphomas that occurred in AIDS patients (Cases 1-3). All three patients were homosexual men presenting extranodal masses in the lungs (Case 1) or skin (Cases 2 and 3), together with the presence of Kaposi's sarcoma (Case 1), primary effusion lymphoma (Case 2), or multicentric Castleman's disease (Case 3). These solid lymphomas exhibited anaplastic large cell morphology and expressed CD30, corresponding to the recent diagnostic criteria of anaplastic large cell lymphoma (ALCL). The chromosomal translocation t(2;5)-associated chimeric protein p80NPM/ALK was not observed in any of these cases. HHV-8 was detected in all of these cases by polymerase chain reaction, immunohistochemistry of HHV-8-encoded ORF73 protein, and in situ hybridization of T1.1. Epstein-Barr virus was detected only in Cases 2 and 3 by in situ hybridization. It is interesting that inoculation of a cell line obtained from a primary effusion lymphoma cell in Case 2 to severe combined immunodeficiency mice produced HHV-8-positive and Epstein-Barr virus-negative tumors in inoculated sites. These tumor cells exhibited phenotypes of ALCL that were identical to the subcutaneous tumor cells of this particular patient. These findings clearly show that HHV-8 can associate with solid lymphomas and that it can take anaplastic large cell morphology. Those lymphomas should be distinguished from the classical ALCL as were defined by the revised European-American classification of lymphoid neoplasms even though morphology and a part of immunophenotype mimic that of classical ALCL.