Study variables in clinical trial specifications are often defined manually, depending on the knowledge and experience of the author rather than on commonly agreed methods and standards. Therefore, we argue that a knowledgebased system can support this task with regard to data quality, consistency and completeness. We present a model based on Semantic Web technology that is flexible enough to represent different granularities and views as well as mapping medical terminologies. That model has been implemented in a software application called Trial Item Manager (TIM).