Lymphomas arising primarily in serosal surfaces have recently been described. Although these "body-cavity" lymphomas usually present as an effusion, the role of cytologic diagnosis has not been fully explored. The authors present seven cases of primary serous lymphoma. All were large cell lymphomas (5 B cell types, 2 T cell types). Three distinct clinical presentations were seen. Four patients had the rapid onset of pleural effusions without an associated mass in the setting of immunodeficiency (3 with AIDS, 1 with Castleman's disease). Cytologic examination of the effusion was positive in all cases for B-cell lymphoma. All four patients died of their disease in less than 6 months. Two other patients with AIDS and T-cell lymphomas both of which were associated with a serosal mass as well as an effusion. Although cytologic examination revealed numerous atypical lymphoid cells, clonality could not be demonstrated. One patient survived for more than 1 year, the other lymphoma was an incidental finding at autopsy. Finally, one B-cell lymphoma arose in a patient with chronic pleuritis. Pleural decortication was required to identify the neoplastic cells. The authors conclude that primary serosal lymphomas have characteristic clinical presentations, and that although cytologic and flow cytometric examination of effusions is necessary for the diagnosis of immunodeficiency associated B-cell lymphomas, tissue biopsy will likely be required in patients with immunodeficiency-associated T-cell lymphomas or lymphomas associated with chronic pleuritis.