© 2015 Elsevier Ireland Ltd. All rights reserved. Background The World Heart Federation criteria for the echocardiographic diagnosis of rheumatic heart disease (RHD) include a category "Borderline" RHD which may represent the earliest evidence of RHD. We aimed to determine the significance of minor heart valve abnormalities, including Borderline RHD, in predicting the future risk of acute rheumatic fever (ARF) or RHD. Methods A prospective cohort study of Aboriginal and Torres Strait Islander children aged 8 to 18 years was conducted. Cases comprised children with Borderline RHD or other minor non-specific valvular abnormalities (NSVAs) detected on prior echocardiography. Controls were children with a prior normal echocardiogram. Participants underwent a follow-up echocardiogram 2.5 to 5 years later to assess for progression of valvular changes and development of Definite RHD. Interval diagnoses of ARF were ascertained. Results There were 442 participants. Cases with Borderline RHD were at significantly greater risk of ARF (incidence rate ratio 8.8, 95% CI 1.4-53.8) and any echocardiographic progression of valve lesions (relative risk 8.19, 95% CI 2.43-27.53) than their Matched Controls. Cases with Borderline RHD were at increased risk of progression to Definite RHD (1 in 6 progressed) as were Cases with NSVAs (1 in 10 progressed). Conclusions Children with Borderline RHD had an increased risk of ARF, progression of valvular lesions, and development of Definite RHD. These findings provide support for considering secondary antibiotic prophylaxis or ongoing surveillance echocardiography in high-risk children with Borderline RHD.