The present study investigates the effect of lexical difficulty, as measured by frequency, on reading comprehension and recall. It also estimates the relationship between vocabulary size, vocabulary depth, reading comprehension, and recall. To this end, 80 English as a foreign language (EFL) undergraduate university students are administered three standardized instruments including two vocabulary tests and a reading comprehension test. The latter comprises two similar passages (in terms of length and topic) one of which is adjusted by replacing 18% of its words by their lowfrequent synonyms, and both passages are estimated lexically to measure their difficulty. Paired samples t-test results show that comprehension and recall are significantly low (p<.001) in the modified passage. This finding further confirms that lexical frequency measure is an effective estimate in determining reading material difficulty. Furthermore, for the second objective, Pearson product-moment analysis reveals a significantly high correlation between size and comprehension, a moderate to low correlation between depth and comprehension, and a moderate correlation between depth and recall tasks. Consequently, the study sugguests estimating the complexity of EFL reading academic material with lexical difficulty measure using frequency criterion to cope with students’ reading deficiencies, and encourage explicit lexical instructions at EFL undergraduate university levels mainly.