There are four usual causes of pain on the right side under the chest: cardiac causes, pulmonary causes, chest wall causes, and rare or uncommon diseases. Common cardiac causes are unstable angina, coronary atherosclerotic heart disease, exertional cardiomyopathy, myocarditis, pericarditis, and cardiac valve disease, all of which can present with pain on the right side of the lower chest. There are also lung diseases, commonly right-sided pneumonia, right-sided pleural effusion, right-sided pneumothorax, predominantly tension pneumothorax, and common chest wall diseases such as right-sided intercostal neuralgia, right-sided osteoarthritis and costochondritis. Rare diseases and rare conditions can also present with pain under the right side and pain behind the chest, such as pulmonary embolism or aortic coarctation. Regardless of the cause of pain on the right side under the chest, it is recommended to visit a hospital quickly to avoid any effects and delays.