It has been stated that malignancy is the most common aetiology of massive pleural effusions. To determine the most frequent causes of massive pleural effusions and to assess the diagnostic yield of different diagnostic procedures and survival, we prospectively studied 1084 patients with pleural effusion. Massive pleural effusions were identified in 121 of 1084 patients (11.2%). Compared with non-massive pleural effusions, massive pleural effusions were significantly more likely to be malignant (53.7% vs. 38.3%, P=0.03) or secondary to cirrhosis (9.9% vs. 2.6%, P=0.0000). On the other hand, massive pleural effusions were significantly less likely to be secondary to infection (10.7% vs. 19.2%, P=0.003) or congestive heart failure (0.8% vs. 6.7%, P=0.03). There was a significant increase in the yield of diagnostic studies in patients with massive malignant pleural effusions (56.9% for cytological studies and 36.9% for biopsies). On the other hand, there was no difference in the diagnostic yield of microbiological and histological studies in the group of tuberculous pleural effusions. In our study population, patients with non-massive malignant pleural effusions had a significantly better survival than those with massive malignant pleural effusions, with a median survival of 8 months (95% confidence interval, 7-9) compared with 5 months (95% confidence interval, 4-6) (P=0.0009). We conclude that malignancy is the most common cause of a massive exudative effusion. Massive malignant pleural effusions are associated with worse survival, independent of age and histologic subgroup, than are non-massive malignant pleural effusions.