The purpose of this study is to examine the extent to which two specific elements of fidelity of implementation (i.e., adherence and quality of delivery) enhanced or constrained the effects of a reading intervention for middle school students experiencing reading difficulties. A total of 281 middle school students with reading difficulties received an intensive reading intervention (i.e., Corrective Reading Decoding strand) delivered by trained school staff (n = 21) at three middle schools. Although students receiving the Corrective Reading Decoding strand made statistically significant (p less than 0.001) improvements in their basic reading skills and passage comprehension overall, variations in student gains were found. Overall fidelity of implementation accounted for 22% and 18% of the variance in the gains in basic reading skills and passage comprehension of middle school students with reading difficulties, respectively (p less than 0.05). Two teacher actions, following the lesson format as designed and reteaching lessons when needed, predicted student basic reading and passage comprehension skills above and beyond other teacher actions (p less than 0.05). Results, implications, and limitations are discussed. (Contains 2 tables and 1 figure.)