At a minimum, high-quality health care is care that does not harm patients, particularly through medical errors. The first step in reducing the large number of harmful medical errors that occur today is to analyze them. We used an actuarial approach to measure the frequency and costs of measurable US medical errors, identified through medical claims data. This method focuses on the analysis of comparative rates of illness, using mathematical models to assess the risk of occurrence and to project costs to the total population. We estimate that the annual cost of measurable medical errors that harm patients was $17.1 billion in 2008. Pressure ulcers were the most common measurable medical error, followed by postoperative infections and by postlaminectomy syndrome, a condition characterized by persistent pain following back surgery. A total of ten types of errors account for more than two-thirds of the total cost of errors, and these errors should be the first targets of prevention efforts.