Tetralogy of Fallot (TOF), formerly known as Steno-Fallot tetralogy, is a congenital heart defect characterized by four specific cardiac defects. Classically, the four defects are:

• pulmonary stenosis, which is narrowing of the exit from the right ventricle;
• a ventricular septal defect, which is a hole allowing blood to flow between the two ventricles;
• right ventricular hypertrophy, which is thickening of the right ventricular muscle; and
• an overriding aorta, which is where the aorta expands to allow blood from both ventricles to enter.

At birth, children may be asymptomatic or present with many severe symptoms. Later in infancy, there are typically episodes of bluish colour to the skin due to a lack of sufficient oxygenation, known as cyanosis. When affected babies cry or have a bowel movement, they may undergo a "tet spell" where they turn cyanotic, have difficulty breathing, become limp, and occasionally lose consciousness. Other symptoms may include a heart murmur, finger clubbing, and easy tiring upon breastfeeding.

The cause of tetralogy of Fallot is typically not known. Risk factors include a mother who uses alcohol, has diabetes, is over the age of 40, or gets rubella during pregnancy.^: 62 It may also be associated with Down syndrome and other chromosomal defects that cause congenital heart defects.

TOF is typically treated by open heart surgery in the first year of life. The timing of surgery depends on the baby's symptoms and size. The procedure involves increasing the size of the pulmonary valve and pulmonary arteries and repairing the ventricular septal defect. In babies who are too small, a temporary surgery may be done with plans for a second surgery when the baby is bigger. With proper care, most people who are affected live to be adults. Long-term problems may include an irregular heart rate and pulmonary regurgitation.

The prevalence of TOF is estimated to be anywhere from 0.02 to 0.04%. Though males and females were initially thought to be affected equally, more recent studies have found males to be affected more than females. It is the most common complex congenital heart defect, accounting for about 10 percent of cases. It was initially described in 1671 by Niels Steensen. A further description was published in 1888 by the French physician Étienne-Louis Arthur Fallot, after whom it is named. The first total surgical repair was carried out in 1954.