This systematic review of 34 studies of malpractice claims in primary care from five countries identified diagnostic errors and medication errors as the most common types of preventable adverse events in ambulatory medicine. Missed and delayed diagnoses—particularly of cancer and myocardial infarction—were also found to be a main source of malpractice lawsuits in another recent study, which was not included in this analysis. Although malpractice claims are an imperfect data source, this study provides important information to help focus efforts to improve patient safety in ambulatory care.