BACKGROUND Increase of left atrial (LA) diameter in trained athletes has been regarded as another component of the "athlete's heart". OBJECTIVE To evaluate the possible impact of competitive training on LA volume and to define reference values of LA volume index in athletes. RESULTS Six hundred fifteen consecutive elite athletes (370 endurance- [ATE] vs 245 strength-trained athletes [ATS]; 385 men; 28.4 +/- 10.2 years, range 18-40 years) underwent a comprehensive transthoracic echocardiography exam. LA maximal volume was measured at the point of mitral valve opening using the biplane area-length method, and corrected for body surface area. LA mild dilatation was defined as a LA volume index between 29 and 33 mL/m(2), while a moderate dilatation was identified by a LA volume index > or =34 mL/m(2). Left ventricular (LV) mass index and ejection fraction did not significantly differ between the 2 groups. Conversely, ATS showed increased body surface area, sum of wall thickness (septum + LV posterior wall), LV circumferential end-systolic stress (ESSc) and relative wall thickness, whereas LA volume index, LV stroke volume and LV end-diastolic volume were greater in ATE. The range of LA volume index was 26 to 36 mL/m(2) (mean 28.2 +/- 9.2) in men and 22 to 33 mL/m(2) (mean 26.5 +/- 7.2) in women (P < .01). LA volume index was mildly enlarged in 150 athletes (24.3%) and moderately enlarged only in 20, all males (3.2%). Mild mitral regurgitation was observed in 64 athletes (10.3%). LA volume index was significantly greater in ATE (P < .01). By multivariate analysis, the overall population type (P < .01) and duration (P < .01) of training and LV end-diastolic volume (P < .001) were the only independent predictors of LA volume index. CONCLUSIONS In a large population of highly trained athletes, a mild enlargement of LA volume index was relatively common and may be regarded as a physiologic adaptation to exercise conditioning.