Purpose: To assess the effectiveness of 38 percent silver diamine fluoride (SDF) in arresting cavitated caries lesions in young U.S. children. Methods: Children 12 to 71 months of age with severe early childhood caries participated in this phase three, multicenter, randomized, placebocontrolled trial. SDF was applied twice (at baseline and six months), and children were followed for eight months. A planned interim analysis of only the six-month primary outcome caries arrest data, for approximately half of the cohort (680 of 1,144 children), was conducted using a generalized estimating equation model, accounting for non-independence among carious lesions within a patient. Results: Five hundred ninety-nine of the 680 participants, with 1,413 lesions, completed the six-month exam. Lesions in the SDF group demonstrated 54 percent arrest versus 21 percent in the placebo (P<0.001). Conclusions: Silver diamine fluoride was effective at arresting active cavitated lesions in this population, leading to the early stop of the trial. Final analyses of all data and other outcomes are currently underway.