Ongoing physical exercise is able to increase skeletal and cardiac muscle mass. Echocardiography and body impedance analysis permit non-invasive evaluation of these two parameters. The aim of this study was to evaluate the effects of training and detraining on the heart and skeletal muscles of professional soccer players. Twenty-one professional athletes (average age 24 +/- 3.5 years) were evaluated during five different phases of their athletic training and compared with 21 age- and height-matched healthy, non-obese sedentary subjects. All subjects underwent measurement of body mass distribution by means of bioelectrical impedance analysis and of left ventricular mass by means of echocardiography. The control group had lower values of lean and cellular body mass, as well as lower left ventricular mass than the professional athletes. Over the 13-month study period, the athletes showed no substantial modifications in fat and muscle mass parameters. Instead, left ventricular mass values increased during the playing season, evidencing physiological hypertrophy after 6 months of competitive activity. No subsequent increases were observed over the next 2 months, and after detraining, left ventricular mass returned to baseline values. We thus conclude that exercise training brings about changes in cardiac mass without producing parallel changes in skeletal muscular mass.