This study aims to use information in general practice records to describe the types, frequency, preventability and severity of harms patients experience due to their healthcare. General practitioners will review three years of records (consultation notes, prescriptions, investigations, referrals, and summaries of hospital care) from 9000 patients registered in 60 general practices. The types of patient harms recorded, their preventability and outcomes will be documented. Both general practices and patients will be randomly selected so that the study's results will apply nationally, after weighting. Equal numbers of small, medium-sized, large, urban and rural general practices will participate in the study because there may be systematic differences in patient harms in these different types of practices. The study's results will be used in the development of an audit tool to help general practices monitor and improve safety, and provide information to help patients, doctors and planners make safer healthcare decisions.