This paper describes the extension and evaluation of Hussey's pattern language for safety-critical user interface development [Hussey and Mahemoff, 1999]. The patterns were updated and new patterns were added by examining existing medical systems. The new patterns were found by generating problem definitions from perceived exemplary solutions. This method of pattern development and its application is described. A qualitative evaluation of the pattern language was performed by designing a safety-critical user interface for a hypothetical radiation therapy machine. Discussion of the design process focuses on the usefulness of our patterns as a design tool for safety-critical user interfaces.