Thirty-seven records of pleural effusion of known cause which occurred in HIV-seropositive patients hospitalized at the Tenon hospital, Paris, between 1985 and 1990 were analyzed retrospectively and divided into two groups: group 1 (n = 21) made up of kaposian effusions, and group 2 (n = 16) of infectious effusions serving as control. Numerous features distinguished group 1 from group 2 pleural effusions: In group 1 the effusion occurred in patients with cutaneous Kaposi sarcoma in 20/21 cases (P less than 0.001), developed slowly (more than 10 days) in 20/21 patients (P less than 0.001), was associated with a specific fever in only 4/21 cases (P less than 0.001), was bilateral in 20/21 cases (P less than 0.001), was regularly associated with bilateral parenchymatous opacities (P less than 0.001) and was haemorrhagic in 18/21 patients (P less than 0.001). Thus, simple semeiological features are sufficient to predict the cause of pleural effusion occurring in HIV-seropositive patients and to guide the diagnostic approach.