We hypothesized that results of previous investigations indicating an increased prevalence of reading disability in boys compared with girls reflected a bias in subject selection. In an epidemiologic sample of 215 girls and 199 boys, we identified two groups of reading-disabled children: research identified and school identified. Results indicated no significant differences in the prevalence of reading disability in research-identified boys compared with research-identified girls in either second (17 [8.7%] of 196 boys; 15 [6.9%] of 216 girls) or third grade (18 [9.0%] of 199 boys; 13 [6.0%] of 215 girls). In contrast, school identification resulted in the classification of 27 (13.6%) of 198 boys and seven (3.2%) of 216 girls in second grade and 20 (10.0%) of 199 boys and nine (4.2%) of 215 girls in third grade. Our data indicate that school-identified samples are almost unavoidably subject to a referral bias and that reports of an increased prevalence of reading disability in boys may reflect this bias in ascertainment. These findings caution against relying solely on schools for identification of reading-disabled children.