A surrogate marker (from the Latin surrogatum, meaning “substitute”) is a substitute value. A surrogate marker is often a value that can be measured quickly and easily, which is why it is often used in clinical trials. A surrogate marker is not directly significant for individual health, however. Surrogate markers are used to find out whether or not a specific intervention might help. One example of a surrogate marker is lowered blood pressure in the prevention of a stroke. Some examinations measure whether a medication lowers high blood pressure and then conclude that using this medication can prevent strokes. But this is an oversimplification. The fact that a medication lowers blood pressure does not mean automatically that it also lowers the risk of a stroke. This is why it is important to carefully check that a surrogate marker accurately reflects the effect the medication has on criteria that are important for patients, such as a stroke. It would be better for the trial not to measure whether the medication lowers blood pressure, but to examine whether it actually lowers the number of strokes.