In the English language, students who read words accurately but have impairments in reading fluency are under‐studied. The associated difficulties they have with comprehending text make it particularly important to delineate effective interventions for these students. Counter to suggestions that these readers need interventions focused on text reading, we examined the effects of a decoding‐focused intervention. The intervention targeted decoding‐related skills, including speeded training on sublexical spelling patterns. We examined the efficacy of this program for students with fluency‐defined disabilities, and compared gains to those for students with accuracy‐defined disabilities. In the initial phase of the program, readers with fluency‐defined disabilities made greater gains in fluency, while readers with accuracy‐defined disabilities made larger gains in word reading accuracy. The mean fluency score for readers with fluency‐defined disabilities came within the average range across the intervention, as did reading comprehension for both groups. Readers' mastery on speeded learning of sublexical spelling patterns predicted unique variance in fluency outcomes, beyond variance accounted for by pre‐test fluency and word reading accuracy. The results support intervention approaches focused on decoding‐related skills for students who have fluency‐defined disabilities and are consistent with theories of reading fluency that identify a role for automaticity with sublexical spelling patterns.