Background The absence (deletion allele [ D ]) of a 287–base pair marker in the ACE gene is associated with higher ACE levels than its presence (insertion allele [ I ]). If renin-angiotensin systems regulate left ventricular (LV) growth, then individuals of DD genotype might show a greater hypertrophic response than those of II genotype. We tested this hypothesis by studying exercise-induced LV hypertrophy. Methods and Results Echocardiographically determined LV dimensions and mass (n=140), electrocardiographically determined LV mass and frequency of LV hypertrophy (LVH) (n=121), and plasma brain natriuretic peptide (BNP) levels (n=49) were compared at the start and end of a 10-week physical training period in male Caucasian military recruits. Septal and posterior wall thicknesses increased with training, and LV mass increased by 18% (all P <.0001). Response magnitude was strongly associated with ACE genotype: mean LV mass altered by +2.0, +38.5, and +42.3 g in II , ID and DD , respectively ( P <.0001). The prevalence of electrocardiographically defined LVH rose significantly only among those of DD genotype (from 6 of 24 before training to 11 of 24 after training, P <.01). Plasma brain natriuretic peptide levels rose by 56.0 and 11.5 pg/mL for DD and II , respectively ( P <.001). Conclusions Exercise-induced LV growth in young males is strongly associated with the ACE I/D polymorphism.