Current knowledge of the impact of training on left ventricular (LV) morphology and function in pre-pubescent children is limited. After ethical approval, 59 pre-pubescent children (mean +/- SD age 10.5 +/- 0.7 years) volunteered for the study. Twenty-five (11 girls) participated in a 12-week progressive, cycle-based aerobic exercise training programme (ET) of three 30-min sessions per week at 80% maximum heart rate (HR) and 34 (15 girls) as matched controls (CON). Pre- and post-training echocardiograms assessed LV structures and function such as LV internal dimension in diastole (LVIDd), LV mass, stroke volume (SV) and early to late LV filling velocity ratio (E:A). Peak VO2 was determined via a modified McMaster protocol. Mixed, two-way ANOVA and multiple linear regression were used to analyse peak VO2 and LV structural/functional data that had been allometrically scaled. A significant interaction for peak VO2 was observed (54 +/- 7 to 55 +/- 6 and 57 +/- 6 to 56 +/- 7 ml/lean body mass (LBM) per min in ET and CON, respectively). A small, but significant, main effect for time was observed for LVIDd over the intervention period (13.9 +/- 1.0 to 14.2 +/- 1.1 and 13.5 +/- 0.9 to 13.8 +/- 1.0 cm.LBM(-0.33) in ET and CON, respectively) that could be attributed to normal growth and development. Similar changes in SV (2.12 +/- 0.45 to 2.24 +/- 0.45 and 1.95 +/- 0.42 to 2.08 +/- 0.44 ml/LBM) and LV mass (2.59 +/- 0.55 to 2.79 +/- 0.69 and 2.45 +/- 0.60 to 2.61 +/- 0.65 g/LBM) were evident (main effect for time p < 0.05). The E:A ratio did not alter in either group. A decrease in resting HR in ET (p < 0.05) suggested a training effect. Multiple regression revealed that post-training resting HR was the only significant predictor of peak VO2 (R2 = 18.2% and 16.9% CON and ET respectively). These data suggest that training in pre-pubescent children does not influence LV morphology and function within the current population. Moreover, the association between LV structure/function and peak VO2 was small. Future work may wish to impose a greater training volume and assess cardiovascular responses in pre-pubescent children.