Aldesleukin causes side effects in almost every organ. Because of these side effects, aldesleukin only can be given to patients who are physically and mentally able to tolerate them. Most of the side effects are due to "capillary leak" which begins immediately after treatment is started. Capillary leak results in the leakage of proteins out of blood. This causes a loss of fluid from the blood, a decrease in the volume of blood, and a decrease in blood pressure. The decrease in blood pressure can be dramatic and even result in death. More than two-thirds of patients require injectable medications to treat the low blood pressure. Other problems associated with capillary leak include congestion in the lungs, difficulty breathing (which can occur in one-half of patients), wheezing, respiratory failure (1 out of every 11 patients), and swelling due to fluid accumulation in various tissues in the body (half of all patients). Abnormal heart rhythms occur in 1 out of every 12 patients, and heart attacks in 1 out of every 50. Bleeding from the stomach, intestines, and kidney or liver damage also can occur. Moreover, between two-thirds and three-fourths of all patients receiving aldesleukin develop kidney damage. Most of the side effects caused by capillary leak begin to resolve a few hours after stopping aldesleukin therapy.