A three-part correlational study examined the explanatory power of the Lexile theory of reading comprehension, which was based on the semantic and syntactic components of prose. Correlations were performed between the item difficulties of nine nationally normed reading comprehension tests and computer generated difficulties which were reported in Lexiles. A correlation of 0.93 was obtained between observed and theoretical scores. A second test was performed in which the rank order of basal series units were correlated with Lexile ratings of text difficulty. A correlation of 0.99 was obtained. A third test was performed in which the correlations between test item difficulties and Lexile ratings were compared with correlations derived from nine measures of readability. Results indicated that while the Lexile equation produced better correlations on average, analysis of variance revealed that the Lexile ratings did not provide a significantly better explanation of the test item difficulties than the readability formulas. Results indicated that the Lexile theory does account for a significant portion of the difficulty of continuous prose and can be used to generate normative and criterion interpretations of a score which would facilitate the direct matching of student abilities with reading materials of appropriate difficulty. (Two figures and 10 tables of data are included; 42 references are attached.) (RS)