This paper reports the results of a research project aimed at studying improvements in English Language Training (ELT) university students' reading comprehension performance by applying the mediations of a dynamic assessment approach to instruction and assessment. In contrast to static assessments, in which correct responses are indicative only of the learners' current ability at a specific moment in time, dynamic assessment concentrates on the learner's errors which are studied in terms of the individual's ongoing development and learning using mediations to promote growth. Lev Vygotsky (1978) is considered the originator of the theoretical framework upon which dynamic assessment is based; it emphasizes the interdependence of learning that leads development and thus the interlocking of instruction and development. In this mixed methods study, dynamic assessment procedures were conducted with 10 ELT university students. Participants took part in a pretest-mediation-posttest design study. The devised pre and posttests had high reliability estimates. The mediation phase included three intervention sessions, each focused on a particular reading comprehension sub skill. The descriptive and analytic analyses of the results reveal dramatic, measurable progress in participants' reading comprehension performance.