In a method and a computer system for screening of medical cases known proven positive and/or known proven normal cases are inserted in the flow of cases during the screening. On a user's option the known cases can be infiltrated in the case stream in accordance with a fixed pattern or on a random basis. When the radiologist makes a misdiagnosis of a known case this is recognized by the system and a corresponding output message is provided to the radiologist. Further the system has a database for tracing all user actions, including the diagnostic findings for the purposes of generating a user action report, quality control and assurance, and support in litigation.