Acute pulmonary eosinophilia (Loeffler's syndrome) is a condition in which lungs become inflamed due to increase in eosinophils. In other words, it can be said that it is a condition in which Eosinpphils, a type of white blood cells, causes inflammation in the lungs if its size increases. It is estimated that 6 people out of 100000 are commonly affected from the condition. Some of the alternatives or other names of the condition are pulmonary infiltrates with eosinophilia, Loeffler syndrome and eosinophilic pneumonia.

Acute pulmonary eosinophilia (Loeffler's syndrome) can be caused by nickel exposure, parasitic infection like ascaris lumbricoides, recent blood transfusion, etc. Allergic reaction caused by drugs such as sulphonamide, etc are also held responsible for causing the condition.

It is counted as rare disorder and many times it becomes difficult to detect its cause. Therefore, its risk can be only minimized by reducing exposure to certain medicines, metals, etc.