Training adaptation of the left ventricle (LV) and it's reversibility following the cessation of training in adults is well known and also studied in children. In the current study we describe the changes in the LV morphology in association with the training season during a 1.5 year follow-up period. 15 elite adolescent swimmers, seven girls and 8 boys with 6 years of swimming history and 20 hr per week training were observed. Their data were compared with 15 age and gender matched nonathletes. LV adaptation was measured with 2D-echocardiography at the baseline preseason and every 3 months, according to the macro cyclic periods of training. Nonathletes were observed at the first and fifth stage of the study. Remarkable LV morphological adaptation has been detected in the swimmers. The greatest LV muscle mass (LVMM: 228 ± 46g) and smallest end-diastolic diameter (LVIDd:44.9 ± 3.4mm) were observed at the end of the second general endurance preparation period (GEP2), but the LVMM/BSA (Rel.LVMM: 85 ± 10g/m) failed to change during the follow-up in athletes. On the basis of our results, we suggest comparing absolute LV dimensions only in studies made at the same training period to avoid bias due to alterations with the training season.