Rheumatic heart disease affects more than 40.5 million people worldwide and results in 306,000 deaths annually. Echocardiographic screening detects rheumatic heart disease at an early, latent stage. Whether secondary antibiotic prophylaxis is effective in preventing progression of latent rheumatic heart disease is unknown. We conducted a randomized, controlled trial of secondary antibiotic prophylaxis in Ugandan children and adolescents 5 to 17 years of age with latent rheumatic heart disease. Participants were randomly assigned to receive either injections of penicillin G benzathine (also known as benzathine benzylpenicillin) every 4 weeks for 2 years or no prophylaxis. All the participants underwent echocardiography at baseline and at 2 years after randomization. Changes from baseline were adjudicated by a panel whose members were unaware of the trial-group assignments. The primary outcome was echocardiographic progression of latent rheumatic heart disease at 2 years.Among 102,200 children and adolescents who had screening echocardiograms, 3327 were initially assessed as having latent rheumatic heart disease, and 926 of the 3327 subsequently received a definitive diagnosis on the basis of confirmatory echocardiography and were determined to be eligible for the trial. Consent or assent for participation was provided for 916 persons, and all underwent randomization; 818 participants were included in the modified intention-to-treat analysis, and 799 (97.7%) completed the trial. A total of 3 participants (0.8%) in the prophylaxis group had echocardiographic progression at 2 years, as compared with 33 (8.2%) in the control group (risk difference, −7.5 percentage points; 95% confidence interval, −10.2 to −4.7; P<0.001). Two participants in the prophylaxis group had serious adverse events that were attributable to receipt of prophylaxis, including one episode of a mild anaphylactic reaction (representing <0.1% of all administered doses of prophylaxis). Among children and adolescents 5 to 17 years of age with latent rheumatic heart disease, secondary antibiotic prophylaxis reduced the risk of disease progression at 2 years. Further research is needed before the implementation of population-level screening can be recommended. (Funded by the Thrasher Research Fund and others; GOAL ClinicalTrials.gov number, NCT03346525.)