Five million people in the U.S. suffer with heart failure, at a cost of $30 billion/year. Heart failure occurs when the heart is damaged and becomes unable to meet the demands placed on it. Unlike some tissues, heart muscle does not regenerate. Human embryonic stem cells grow and divide indefinitely while maintaining the potential to develop into many tissues of the body, including heart muscle. They provide an unprecedented opportunity to both study human heart muscle in culture in the laboratory, and advance cell-based therapy for damaged heart muscle. During the first year of CIRM support, we have developed methods for identifying and isolating specific types of human embryonic stem cells, and stimulating them to become human heart muscle cells. We are currently working to determine the best methods and timing for delivering these cells into the hearts of mice that have had a heart attack. This research will identify those human embryonic stem cells that are best at repairing damaged heart muscle, thereby treating or avoiding heart failure.