Of the 1.5 million people diagnosed with pleural effusion in the USA annually, ∼178 000 undergo thoracentesis. While it is known that malignant pleural effusion portends a poor prognosis, mortality of patients with nonmalignant effusions has not been well studied. This prospective cohort study evaluated 308 patients undergoing thoracentesis. Chart review was performed to obtain baseline characteristics. The aetiology of the effusions was determined using standardised criteria. Mortality was determined at 30 days and 1 year. 247 unilateral and 61 bilateral thoracenteses were performed. Malignant effusion had the highest 30-day (37%) and 1-year (77%) mortality. There was substantial patient 30-day and 1-year mortality with effusions due to multiple benign aetiologies (29% and 55%), congestive heart failure (22% and 53%), and renal failure (14% and 57%, respectively). Patients with bilateral, relative to unilateral, pleural effusion were associated with higher risk of death at 30 days and 1 year (17% versus 47% (hazard ratio (HR) 2.58, 95% CI 1.44-4.63) and 36% versus 69% (HR 2.32, 95% CI 1.55-3.48), respectively). Patients undergoing thoracentesis for pleural effusion have high short- and long-term mortality. Patients with malignant effusion had the highest mortality followed by multiple benign aetiologies, congestive heart failure and renal failure. Bilateral pleural effusion is distinctly associated with high mortality.