Factors in reading comprehension relating to English as a second language (ESL) are considered, focusing on what contributes to text comprehensibility (grammatical structure or density of new vocabulary), the effect of a learner's English language proficiency on use of grammatical words for comprehension, and the possible interaction of these factors with the first language background of the reader. Five hypotheses are proposed, the central one arguing that content words are more essential for comprehension than function words, and the other four investigating the interactive effects of the word-class distinction with the density of unknown words, syntactic complexity, learner English proficiency, and native language background. Results of testing the hypotheses with 33 Spanish-speaking and 16 Arabic-speaking ESL students are reported. It is concluded that knowledge of content words is the most essential knowledge in ESL reading comprehension, and that grammar is secondary, especially if the native language is similar to English. In addition, a density of new vocabulary of about one new word to nine familiar words is suggested as tolerable for ESL learners. A slightly higher dependency on function words is found in low English proficiency students. Implications for the practice of language across the curriculum are discussed. A 52-item reference list is appended. (MSE)