The current study meta-analyzed 27 effects from 21 studies to determine the effect assessment of text difficulty had on reading fluency interventions, which resulted in an overall weighted effect size (ES) = 0.43 (95% CI = [0.25, 0.62], p < 0.001). Using reading passages that represented an instructional level based on accuracy criteria led to a large weighted effect of ES = 1.03, 95% CI = [0.65, 1.40], p < 0.01), which was reliably larger (p < 0.05) than that for reading fluency interventions that used reading passages with an instructional level based on rate criteria (weighted ES = 0.29, 95% CI = [0.07, 0.50], p < 0.01). Using reading passages based on leveling systems or those written at the students' current grade level resulted in small weighted effects. The approach to determining difficulty for reading passages used in reading fluency interventions accounted for 11% of the variance in the effect (p < 0.05) beyond student group (no risk, at-risk, disability) and type of fluency intervention. The largest weighted effect was found for students with reading disabilities (ES = 1.14, 95% CI = [0.64, 1.65], p < 0.01).