Adaptive Hypermedia Systems can be developed to accommodate a variety of individual differences, including learning style and cognitive style. This study investigates the hypothesis that adaptive hypermedia accommodating cognitive styles can be beneficial for the observed learning outcomes. A prototype system, designed to be adapted to individual cognitive styles, was developed as a case study. In order to evaluate the effectiveness of the prototype system, an empirical study was conducted. This paper presents the results of the summative evaluation of the system. Statistical analyses indicated that students in the experimental group performed significantly better than students in a control group. These findings indicate that student performance is mainly affected by adaptivity based on individual cognitive styles.