Lymphomas were documented in pleural effusions or ascites in 18 human immunodeficiency virus-positive (HIV+) patients. Eleven of 12 with clinical data had acquired immunodeficiency syndrome before the diagnosis of lymphoma. In 13 of 15 with data available, a body cavity was the site of initial presentation of lymphoma. Cytological subtypes were large cell immunoblastic, n = 7; large cell anaplastic, n = 6; and large cell NOS, n = 5. The high incidence of anaplastic large cell lymphoma and the conspicuous absence of Burkitt's lymphoma differ strikingly from HIV-associated lymphomas generally. Immunophenotypically, two cases were B-cell (CD19/20+, sIg+, CD/5-), one was T-cell (CD3+, CD5+, CD4+, CD8-, CD19/20-, sIg-), and 15 were null (CD45+, HLA-DR+ CD19/20-, sIg-, CD3/5-). This 83% incidence of null immunophenotype contrasts sharply with a 9% incidence among 35 tissue-based lymphomas in HIV+ patients that were similarly studied and a 0% null immunophenotype among 11 lymphomatous effusions in patients without HIV risk factors. Seven of the 18 HIV-associated lymphomas expressed CD30. Four of five cases with null immunophenotype showed Ig heavy-chain gene rearrangement, two had clonal Epstein-Barr virus integration, and none had MYC protooncogene rearrangement. These cases belong to a subgroup of high-grade HIV-associated lymphomas that occur in the setting of profound immunosuppression in which immunoblastic morphology predominates and MYC rearrangement is encountered only infrequently.