Purpose: This study examined whether a particular type of therapy (Broad Target Speech Recasts, BTSR) was superior to a contrast treatment in facilitating speech comprehensibility in conversations of students with Down syndrome who began treatment with initially high verbal imitation. Method: We randomly assigned 51 5- to 12-year-old students to either BTSR or a contrast treatment. Therapy occurred in hour-long 1-to-1 sessions in students' schools twice per week for 6 months. Results: For students who entered treatment just above the sample average in verbal-imitation skill, BTSR was superior to the contrast treatment in facilitating the growth of speech comprehensibility in conversational samples. The number of speech recasts mediated or explained the BTSR treatment effect on speech comprehensibility. Conclusion: Speech comprehensibility is malleable in school-age students with Down syndrome. BTSR facilitates comprehensibility in students with just above the sample average level of verbal imitation prior to treatment. Speech recasts in BTSR are largely responsible for this effect.