Reading impairment is more common in males, but the magnitude and origin of this gender difference are debated. In a large-scale study of reading impairment among 491,103 beginning second-graders, gender differences increased with greater severity of reading impairment, peaking at a ratio of 2.4:1 for a broad measure of fluency and a ratio of 1.6:1 for a narrow measure of decoding. Results from three tests indicate that gender differences in reading impairment are attributable primarily to male vulnerability rather than ascertainment bias. Correspondence between identification as an impaired reader by our study criteria and school identification as learning disabled was poor overall and worse for girls: Only 1 out of 4 boys and 1 out of 7 girls identified as reading impaired in our study was school identified as learning disabled.