To investigate the etiology of pleural effusions in our region, we undertook a prospective study of patients with this condition in our centers. During a 5-year period, we studied 642 pleural effusion patients aged 57.1 +/- 21.1 years, of whom 401 were men aged 56.5 +/- 21 years and 241 were women aged 57.8 +/- 21.4 years; the male/female ratio was 1.6:1. The most frequent cause of pleural effusion was tuberculosis (25%), followed by neoplasia (22.9%) and congestive heart failure (17.9%). The etiology of 48 cases (7.5%) remained uncertain. In the neoplastic effusion group, the most frequent locations of the primary tumor were lung (32.6%), breast (11.5%), lymphoma (10.8%), and ovary (7.5%); in 21 cases (14.3% of the neoplastic group), it was not possible to identify the primary tumor. The 111 patients aged younger than 40 years with tuberculous effusions made up 69.4% of tuberculous effusion cases and the same percentage of patients younger than 40 years; the proportion of effusions that were tuberculous peaked in the 11- to 30-year-old age group and declined steadily thereafter. Of the patients with neoplastic effusions, 83% were older than 50 years; the proportion of effusions that were neoplastic rose steadily from zero in the 0- to 30-year-old age group to a peak among 60- to 70-year-olds. The age-wise distribution of effusions secondary to congestive heart failure was similar to that of neoplastic effusions. Of the effusions secondary to congestive heart failure, 86% (99/115) affected the right pleura or both, and 83% of effusions secondary to pulmonary thromboembolism (15/18) affected the right side. Neoplastic, tuberculous, parapneumonic, empyematous, and other exudative effusions showed no preference for either side. Of the 97 bilateral effusions, 77 (79.4%) were secondary to heart failure (59, 60.8%) or neoplasia (18, 18.6%). We conclude that in our region, the most frequent cause of pleural effusion is tuberculosis, followed by neoplasia and congestive heart failure. We suggest that all those interested in pleural disease should determine the etiologic pattern of pleural effusion in their region with a view to the adoption of regionally optimized diagnostic and therapeutic attitudes.