This study examined the effectiveness of a researcher-provided intervention with fourth graders with significant reading difficulties. The intervention emphasized multisyllable word reading, fluent reading of high-frequency words and phrases, vocabulary, and comprehension. To identify the participants, 1,695 fourth-grade students were screened using the Gates-MacGinitie Reading Test, and those whose standard scores were 85 or lower were included in the study (N = 483). Participants were randomly assigned (2:1) to receive either researcher-provided intervention (n = 323) or intervention provided by school personnel (business as usual, BAU) (n = 161). Findings revealed no statistically significant differences between students in the researcher-provided intervention and BAU groups. Using effect sizes as an indicator of impact, students in the researcher-implemented treatment generally outperformed students in the school-implemented treatment (BAU). Examining growth in standard scores, both groups made significant gains in reading outcomes with standard score growth from pretest to posttest of 3 standard score points on decoding, 5 on fluency, and 2.0 to 7 standard score points on reading comprehension measures.