Patient safety in ambulatory care is garnering more attention, in recognition of the fact that most health care is delivered in the outpatient setting. However, little data exists on the incidence of ambulatory errors. This survey of patients attending primary care clinics found that nearly 1 in 3 patients reported experiencing a mistake or diagnostic error by a physician in the previous 10 years, with 14% of patients having changed physicians as a result. An accompanying editorial discusses the challenges of improving ambulatory safety and suggests strategies for addressing common sources of error, including improved mechanisms for communication between providers, computerized provider order entry, and systematic strategies to identify missed or delayed diagnoses.