Lone atrial fibrillation (AF) can cause functional mitral regurgitation (MR), commonly referred to as “atrial functional MR (AFMR).” This type of MR has recently received much attention as an important cause of heart failure, and it represents a considerable therapeutic target in heart failure patients with AF. Mitral annular dilatation due to left atrial (LA) dilatation can be recognized as an original cause of AFMR, whereas the exact cascade of AFMR etiologies has not been established. AFMR is typically classified as Carpentier type I, and is likely to have a central jet. In contrast, a proportion of AFMR is classified as a combination of Carpentier type I for a flattened anterior mitral leaflet and Carpentier type IIIb for a tethered posterior mitral leaflet and is likely to have an eccentric jet directed toward the LA posterior wall. The traditional functional MR occurring in patients with left ventricular (LV) dilatation and/or systolic dysfunction, which is classified as Carpentier type IIIb, has since been designated “ventricular functional MR (VFMR)” to distinguish it from AFMR. Traditional VFMR, newly recognized AFMR, and their etiologic relations to LV/LA size and function are discussed in this review article.