The aim of this paper is to evaluate the applicability of Vygotsky's sociocultural theory and the concept of dynamic assessment to the problem of assessing reading comprehension in English as a third language (L3). The study was conducted with immigrant students from Ethiopia studying at pre-academic centers in Israel. The assessment procedure included a pre-test, mediation, and post-test. The pre-test was based on a standard reading comprehension placement exam. Based upon the pre-test material, a very detailed analysis of the required pre-existing knowledge and necessary strategies was then mediated to the students. The post-test was identical to the pre-test regarding structure, requirements in terms of pre-existing knowledge and strategies, length, etc., but with different content. The results demonstrated: (1) A dynamic assessment procedure significantly improved the text comprehension performance of L3 students; (2) The dynamic assessment procedure added information regarding students' learning potential over and beyond their initial performance level; (3) The learning potential profile of immigrants from Ethiopia is somewhat different from that of non-immigrant students. (Contains 2 figures and 1 table.)