The objective of our study was to evaluate the prevalence of pleural effusions in patients with the acquired immunodeficiency syndrome, to correlate these effusions with any concomitant pulmonary diseases and to evaluate the role of cytologic examination in the diagnosis of the effusions. Twenty-eight of 389 (7.2%) human immunodeficiency virus-infected patients had pleural effusions and 27 of the 28 were suffering from concomitant pulmonary diseases. Those diseases were bacterial pneumonia (9), mycobacterial infection (7), non-Hodgkin's lymphoma (4) and Kaposi's sarcoma (2). Pneumocystis carinii pneumonia was diagnosed in two patients, and cytomegalovirus pneumonitis and pulmonary aspergillosis and small cell carcinoma in one patient each. Cytologic examination of pleural effusions provided conclusive diagnoses of mycobacterial infection in 2 of the 7 patients, of non-Hodgkin's lymphoma in 4 and of P carinii infection in 2.