In patients with cardiomegaly and signs and symptoms compatible with CHF, unilateral right-sided or bilateral pleural effusions of similar size are likely to be due to left-sided CHF. Isolated right ventricular failure or chronic pulmonary hypertension is not usually associated with pleural effusions, and unrecognized or new-onset left ventricular dysfunction and other causes should be considered when a patient with cor pulmonale presents with a pleural effusion. Unilateral left-sided pleural effusions with cardiomegaly may be due to pericardial disease. Current hypotheses do not adequately explain the laterality of effusions in CHF or pericardial disease. Clinical and radiographic correlation is always required; however, the associations described occur often enough to make them useful in day-to-day clinical practice. When ascribing pleural effusions to CHF, clinicians must be sure the clinical signs and history "fit the picture," because pneumonia and pulmonary embolism may also cause pleural effusions in patients with heart failure. Typical pleural effusions in patients with uncomplicated CHF (demonstrated by small to medium-sized effusions and the absence of fever, leukocytosis, pleuritic chest pain, or marked asymmetry in bilateral effusions) do not require routine diagnostic thoracentesis for evaluation. A reasonable approach in such cases is treatment of the underlying CHF and follow-up radiography to monitor for resolution of the effusions. Prompt diagnostic thoracentesis is indicated whenever atypical features are present and other diagnoses are under consideration.