Validity is a central issue in portfolio-based assessment. This empirical study used a quantitative approach to analyse the validity of the inferences drawn from a disciplinary course work portfolio assessment comprising profession-specific and learning competencies. The study also examined the problems involved in the development of the portfolios and the features of the portfolio design that had a positive effect on validity. Teachers evaluated the performance of 25 first-year university students who produced portfolios for an obligatory course in a fine arts degree. The study data comprised the 1,300 scores obtained by the students. To assess the adequacy of the portfolio in its educational context, both the students' workload and their final grades were examined. The examination of the final grades and factor analyses confirmed the validity of portfolio-based assessment but also revealed the difficulty of encouraging students to reflect on the learning process, especially where higher-order learning outcomes were concerned. The students considered that the 25.49 % increase in hours of autonomous work made the workload excessive. With a Cronbach "a" above 0.90, however, the use of a well-structured portfolio and a descriptive and detailed assessment system helped to compensate for the complexity of the learning outcomes and the students' lack of familiarity with portfolios.