Standardized patients were sent to general practitioners who use the patient list system in Trondheim in order to register daily clinical practice without the patient being unmasked. The authors explain what a standardized patient is, how they are taught to present a disease, and how they report on the consultation in a valid and reliable way. They also describe how the standardized patients were introduced into the doctors' patient list system. The doctors were informed about the project in advance. Twenty-three doctors were visited twice and one doctor was visited once by a standardized patient. At two of the visits the patient was unmasked. The conclusion is that the use of standardized patients is a valid, reliable and practical method for quality assurance in general practice in Norway.