Many studies have attempted to relate genetic variants of components of the renin-angiotensin system to complex diseases such as essential hypertension, cardiovascular disease and progressive renal failure. The angiotensin II type 1 receptor (AT1R) gene is an important example of this approach. Many polymorphisms of the AT1R gene have been identified, but the A1166-->C polymorphism has been the most extensively studied. The physiological significance of this polymorphism is uncertain because of its location in the 3'-untranslated region of the gene. The present review summarizes association studies of the AT1R gene, focusing on clinical end-points and physiological responses.