Large and moderate pleural effusions can differ in terms of fluid volume, etiology, symptoms, and imaging manifestations. 1. Fluid volume: large pleural effusion is usually greater than 1000 ml, while intermediate pleural effusion is between 500 and 100 ml. 2. Etiology: pleural effusion caused by malignant tumors is usually large, while tuberculous pleural effusion is usually medium or small, and pleural effusion caused by congestive heart failure is usually in medium volume. 3. Symptoms: Medium amount of pleural effusion compresses the lung tissue and symptoms such as chest tightness and shortness of breath may appear; large amount of pleural effusion compresses the cardiopulmonary tissues more heavily, and symptoms such as chest tightness, shortness of breath, palpitation and so on are more serious, and symptoms such as sedentary respiration and cyanosis may appear. 4. Imaging manifestations: X-ray, CT and other examinations show that the upper edge of medium-volume pleural effusion is above the plane of the 4th rib and below the plane of the 2nd rib. A large amount of pleural effusion with the upper edge above the plane of the second rib may be accompanied by widening of the rib space and decrease of the diaphragm. It is recommended that patients with pleural effusion go to the hospital as soon as possible to avoid delaying their condition.