This study examined the relationships of 3 levels of reading fluency--the individual word, the syntactic unit, and the whole passage--to reading comprehension among 278 5th graders heterogeneous in reading ability. Hierarchical regression analyses revealed that reading fluency at each level related uniquely to performance on a standardized reading comprehension test in a model including inferencing skill and background knowledge. The study supports an automaticity effect for word recognition speed and an automaticity-like effect related to syntactic processing skill. In addition, hierarchical regressions using longitudinal data suggest that fluency and reading comprehension have a bidirectional relationship. The discussion emphasizes the theoretical expansion of reading fluency to 3 levels of cognitive processes and the relations of these processes to reading comprehension. (Contains 7 tables.)