In many of the accidental exposures described in this report, a single cause cannot be identified. Usually, there was a combination of factors contributing to the accident, e.g., deficient staff training, lack of independent checks, lack of quality control procedures, and absence of overall supervision. Such combinations often point to an overall deficiency in management, allowing patient treatment in the absence of a comprehensive quality assurance programme. Factors common to many accidents are identified and discussed in detail. The use of radiation therapy in the treatment of cancer patients has grown considerably and is likely to continue to increase. Major accidents are rare, but are likely to continue to happen unless awareness is increased. In this report, explicit recommendations on measures to prevent radiotherapy accidents are given with respect to regulations, education, and quality assurance.