The identification and reduction of medical error has become a major priority for all health care providers, including primary care. Understanding the frequency and nature of medical error in primary care is a first step in developing a policy to reduce harm and improve patient safety. There has been scant research into this area. This review had two objectives; first, to identify the frequency and nature of error in primary care, and, secondly, to consider the possible causes for the diversity in the stated rates and nature of error in primary care. Literature searches of English language studies identified in the National Patient Safety Foundation bibliography database, in Medline and in Embase were carried out. Studies that were relevant to the purpose of the study were included. Additional information was obtained from a specialist medico-legal database. Studies identified that medical error occurs between five and 80 times per 100000 consultations, mainly related to the processes involved in diagnosis and treatment. Prescribing and prescription errors have been identified to occur in up to 11% of all prescriptions, mainly related to errors in dose. There are a wide variety of definitions and methods used to identify the frequency and nature of medical error. Incident reporting, systematic identification and medico-legal databases reveal differing aspects, and there are additional perspectives obtained from GPs, primary health care workers and patients. An understanding of the true frequency and nature of medical error is complicated by the different definitions and methods used in the studies. Further research is warranted to understand the complex nature and causes of such errors that occur in primary care so that appropriate policy decisions to improve patient safety can be made.