The genesis of the fourth heart sound (S4) is commonly related to the rapid set in vibration of the left ventricular walls, resulting from the rapid inflow of the blood due to the atrial contraction (Nishimura et al., 1989). S4 can be recorded in normal young subjects as an expression of physiologic atrial dynamics but it is more common in pathologic conditions characterized by decreased ventricular distensibility (Tavel, 1978). Employing the spectral analysis of heart sounds (Longhini et al., 1979; Longhini et al., 1981; Aubert et al., 1984) we searched for the relationship between different components of the frequency spectrum of S4 and various echocardiographic parameters, with the aim of identifying the cardiac structures involved in its genesis (Longhini et al., 1989; Baracca et al., 1991).