Issue addressed: Use of the Internet for health promotion is increasing; however the lack of published research regarding website usability suggests that health promotion websites are being developed without formal evaluation and consultation of their users. The purpose of this study was to conduct usability testing of an existing health promotion website to inform modifications to improve the website, and to identify common usability themes that should be addressed by organisations developing or maintaining a health promotion website. Methods: A combination of qualitative and quantitative techniques were implemented during the usability testing sessions to gather data from users while completing tasks on the website. Techniques included performance measures (time taken), direct observation (participant observation) and subjective user preferences (questionnaire and interview). Results: Improvements to the website were measured in terms of reduced problems reported, reduced time taken to complete tasks and increased subjective reports. In addition, seven usability themes emerged from the data. These were identified as design, feedback, format, instructions, navigation, terminology and learnability. Conclusions: This study demonstrates the application of usability testing to the design and modification of a health promotion website and illustrates the areas, or themes that can be used as a framework for testing and modification.