We investigated the hierarchical organization of three core processes involved in written language comprehension: (1) letter processing, (2) lexical processing, and (3) syntactic processing. Because of the hypothesized hierarchical organization, syntactic processing should be influenced by lexical processing which in turn should be influenced by letter-level processing. To test this hypothesis, skilled readers performed three different binary-decision tasks: alphabetic (AD), lexical (LD), and grammatical (GD). AD, LD, and GD were respectively used as a marker of efficiency for letter, lexical, and syntactic processing. Results showed that the relation between two hierarchically adjacent processes (AD/LD and LD/GD) is stronger than for non-adjacent process (AD/LD). Moreover, better performance in both the alphabetic and lexical decision tasks led to better performance in grammatical decision making. These results provide support for a hierarchically organized architecture of written language comprehension involving parallel, cascaded processing operating between letters and words and also between words and sentences.