We know that knowledge of word structure--morphology--relates to reading, but there is limited research on its unique contribution to reading comprehension, especially with students in middle and high schools and with the nesting of students within classrooms taken into account. In this study with 4780 students in grades 3 to 10, we examined how students' winter scores in morphological knowledge, spelling, text reading efficiency, and reading comprehension predicted spring scores in reading comprehension. Reading comprehension was measured by a computer-adaptive, interim reading assessment. Bivariate relations were examined and a strong relation between morphological knowledge and reading comprehension was observed in all grades except grade 10. Multilevel analyses showed that morphological knowledge added 2%-9% unique variance beyond the autoregressor in predicting spring reading comprehension at the student level and that over 90% of the large variability between classrooms was explained by performance on these component skills. Educational implications were discussed. (Contains 4 tables and 1 figure.)