Reading is important for children's success in school and beyond, yet many adolescents fail to reach expected levels of proficiency. This highlights the need to better understand the factors that influence reading effectiveness over time, including genes and environment. Greater expression of genetic influence on first- and second-grade reading fluency has been observed in higher quality classroom reading environments. To what degree this early environment continues to influence genetic and other environmental influences on later reading is unknown and was tested in this study.The quality of the early classroom reading environment was approximated by gains in oral reading fluency (ORF) across the school year among first- or second-grade classmates of 546 MZ and 1,016 DZ twin children (mean age = 7.13 years; SD = 0.45) who had reading comprehension scores from a state-wide mandatory test in school year 2013-2014 when most twin pairs were in seventh to tenth grade (mean age = 14.41; SD = 1.13) in a variable called Class ORF Gain. Biometrical models were fit to the data to assess whether Class ORF Gain moderated the genetic, shared environmental and/or nonshared environmental variance associated with adolescent reading comprehension.Class ORF Gain moderated shared environmental influences on reading comprehension 6-9 years later. When early classroom reading gains were poor, variability in reading comprehension in adolescence was high and was associated largely with shared environmental influences. When early classroom reading gains were good, overall and shared environmentally influenced variability in adolescent reading comprehension was lower so that genetic influences were most relevant in explaining that variability.Our findings suggested that classroom reading environment experienced when children were learning to read had a lasting influence on the factors underlying variability in later reading effectiveness.