There is a need for valid methods to assess the readiness for clinical practice of medical graduates. This study evaluates the validity of Utrecht Hamburg Trainee Responsibility for Unfamiliar Situations Test (UHTRUST), an authentic simulation procedure to assess whether medical trainees are ready to be entrusted with unfamiliar clinical tasks near the highest level of Miller's pyramid. This assessment, in which candidates were judged by clinicians, nurses and standardized patients, addresses the question: can this trainee be trusted with unfamiliar clinical tasks? The aim of this paper is to provide a validity argument for this assessment procedure. We collected data from various sources during preparation and administration of a UHTRUST-assessment. In total, 60 candidates (30 from the Netherlands and 30 from Germany) participated. To provide a validity argument for the UHTRUST-assessment, we followed Kane's argument-based approach for validation. All available data were used to design a coherent and plausible argument. Considerable data was collected during the development of the assessment procedure. In addition, a generalizability study was conducted to evaluate the reliability of the scores given by assessors and to determine the proportion of variance accounted by candidates and assessors. It was found that most of Kane's validity assumptions were defendable with accurate and often parallel lines of backing. UHTRUST can be used to compare the readiness for clinical practice of medical graduates. Further exploration of the procedures for entrustment decisions is recommended.