Objectives Little is known about the fate of untreated moderate aortic regurgitation after mitral valve replacement for rheumatic heart disease. The aim of this study was to compare the progression to significant aortic valve disease of untreated moderate and mild aortic regurgitation after mitral valve replacement for rheumatic heart disease. Methods Among patients who underwent mitral valve replacement for rheumatic heart disease without concomitant aortic valve intervention at our institution between January 2005 and December 2017, we selected 338 patients with pure aortic regurgitation (273 patients with mild and 65 patients with moderate aortic regurgitation). Results After a median follow-up of 10.0 years, 16 patients developed significant aortic valve disease. Freedom from significant aortic valve disease at 3 years, 8 years, and 14 years was 100%, 97.8 ± 1.1%, and 93.5 ± 2.3% in patients with mild aortic regurgitation, and 98.5 ± 1.5%, 94.5 ± 3.2%, and 80.6 ± 7.5% in patients with moderate aortic regurgitation (log-rank test: p = 0.007). Seven patients with mild aortic regurgitation and two patients with moderate aortic regurgitation underwent aortic valve replacement. Conclusions After mitral valve replacement for rheumatic heart disease, untreated moderate aortic regurgitation progressed more frequently to significant aortic valve disease compared to untreated mild aortic regurgitation. A marked difference in freedom from significant aortic valve disease occurred after eight years. Aortic valve replacement was rarely needed.