Objective: To describe the range of pathology causing pleural effusions in HIV infected patients with acute respiratory episodes and to attempt to identify whether any associated radiological abnormalities enabled aetiological discrimination. Methods: Prospective study of chest radiographs of 58 consecutive HIV infected patients with pleural effusion and their microbiological, cytological, and histopathological diagnoses. Results: A specific diagnosis was made in all cases. Diagnoses were Kaposi's sarcoma, 19 patients; parapneumonic effusion, 16 patients; tuberculosis, eight patients; Pneumocystis carinii pneumonia, six patients; lymphoma, four patients; pulmonary embolus, two patients; and heart failure, aspergillus/leishmaniasis, and Cryptococcus neoformans, one case each. Most effusions (50/58) were small. Bilateral effusions were commoner in Kaposi's sarcoma (12/19) and lymphoma (3/4) than in parapneumonic effusion (3/16). Concomitant interstitial parenchymal shadowing did not aid discrimination. A combination of bilateral effusions, focal air space consolidation, intrapulmonary nodules, and/or hilar lymphadenopathy suggests Kaposi's sarcoma. Unilateral effusion with focal air space consolidation suggests parapneumonic effusion if intrapulmonary nodules are absent: if miliary nodules and/or mediastinal lymphadenopathy are detected, this suggests tuberculosis. Conclusions: A wide variety of infectious and malignant conditions cause pleural effusions in HIV infected patients, the most common cause in this group was Kaposi's sarcoma. The presence of additional radiological abnormalities such as focal air space consolidation, intrapulmonary nodules, and mediastinal lymphadenopathy aids aetiological discrimination.