The debate over medical malpractice litigation continues unabated in the United States and other countries. Advocates of tort reform, including members of the Bush administration, lament the burden of “frivolous” malpractice lawsuits and cite them as a driving force behind rising health care costs. (A frivolous claim is one that “present[s] no rational argument based upon the evidence or law in support of the claim.”) Plaintiffs’ attorneys refute this charge, countering that contingency fees and the prevalence of medical errors make the pursuit of meritless lawsuits bad business and unnecessary. Previous research has established that the great majority of patients who sustain a medical injury as a result of negligence do not sue. However, the merit of claims that are brought, and the ability of the malpractice system to resolve them appropriately, remain much more controversial. If frivolous claims are common and costly, they may be a substantial source of waste in the health care and legal systems. We investigated the merits and outcomes of malpractice litigation using structured retrospective reviews of 1452 closed claims. The reviews included independent assessments of whether the claim involved injury due to medical error. Our aim was to measure the prevalence, costs, outcomes, and distinguishing characteristics of claims that did not involve identifiable error.