The development of customized application programs by the end users of data management systems requires special consideration when computer validation is being discussed. These customized programs are produced to capture, tabulate, and analyze data within the context of a specific clinical trial. They may be numerous and in a busy department may be produced on a frequent basis. This paper considers the application of computer validation principles—EC GCP requirements and the principles of the ISO 9000 quality assurance standards—to the validation of these programs. The validation procedure needs to be stringent but straightforward and its documentation practical in order to ensure compliance. It should ensure audit friendly documented evidence that proper validation has taken place. Validation of these end user developed applications is of particular importance in assuring accurate data transformation. It can be effective and is proactive in providing for high quality clinical trial report production.