This article describes the results of an investigation into the suitability of methodology based on UML in an IEC 61508 development project that developed SIL products. Feedback was gathered from project members through interviews and questionnaires. The results show that the method led to improvements in some aspects of the development process that are particularly important in the development of SIL products. These improvements were related to documentation, traceability from requirements to code, and the quality of the code. The results also show, however, that the opinions of the project members differed a lot, and consequently that it is difficult to define one method with which a large number of developers with different backgrounds and mind sets will be content.