Two studies report on the development of the Kidney Disease Questionnaire (KDQ) as a test for measuring patient knowledge about end-stage renal disease and its treatment. The KDQ is available in a 26-item version or as two parallel 13-item tests. Psychometric evaluations indicate that all versions show high levels of reliability. Initial validity tests are also promising. The KDQ is able to discriminate individuals well informed about kidney disease and its treatment from those who are not so well informed. It is also sensitive to the effects of an experimental education program and to ESRD-related knowledge that is acquired as a result of starting dialysis. Data and issues related to the administration, readability, demographic correlates, and a French translation of the KDQ are also presented and discussed.