The following reasons may cause the symptom of small amount of pleural effusion on the right side: 1) Infectious diseases of lungs, such as mycoplasma pneumonia, tuberculosis, pulmonary mycosis, etc., because the interstitial tissues of lungs and the surrounding tissues are stimulated by persistent inflammation, secretion will be exuded, which will lead to the symptom of small amount of pleural effusion on the right side. Tumor lesions, such as lung adenocarcinoma, alveolar carcinoma, lung fibroma, etc. In case of occupying lesions in lung or surrounding organs and tissues, normal cells and tissues will be infiltrated by tumor cells, which will cause the symptoms of malignant pleural effusion. Third, rheumatic diseases, cardiogenic diseases, connective tissue diseases may cause the symptoms of localized pleural effusion. It is recommended to go to the regular hospital to receive a more detailed examination to identify the specific causes of the disease and then carry out symptomatic treatment.