This article reports the results of a study on the relationship between second language vocabulary breadth, vocabulary depth, and reading comprehension. A special aim was to discover the role of vocabulary depth in the prediction and explanation of L2 learners’ reading comprehension. Two proficiency levels, intermediate and advanced, were compared. Vocabulary breadth was measured with the Vocabulary Size Test (Nation & Beglar, 2007), vocabulary depth with the Vocabulary Knowledge Scale (Wesche & Paribakht, 1996), and reading comprehension with a test compiled of sections from two former matriculation examination tasks. The three constructs showed strong positive correlations in both groups of subjects. However, based on the results, vocabulary breadth and depth seem to be stronger predictors of reading comprehension skills in lower levels of proficiency than on the advanced level.