The purpose of this study was to examine the effect of 2 school years (21 months) of a twice-weekly resistance training program on stature, muscle strength, and self-concept among prepubertal boys. The experimental group (E, n = 27) aged 9.2 ± 0.3 yrs, participated in progressive resistance training, while the control group (C, n = 22) aged 9.4 ± 0.3 yrs, participated in standard physical education classes (as advised by the Ministry of Education). Training sessions included 1–4 sets of 3–6 exercises, with 5–30 repetitions/set. The load ranged between 30% and 70% 1RM. No differences were observed in the gain in body height between groups. Muscle strength increased significantly more in E (e.g., knee extensors: 0.51 ± 0.13 to 0.77 ± 0.16 kg/kg body mass), compared with C (0.34 ± 0.12 to 0.54 ± 0.11 kg/kg body mass). One minor injury was reported throughout the study. Initial scores of self-concept were high in both groups, with no training effect. The results demonstrate that among prepubertal boys, a twice-weekly low-to-moderate-intensity resistance training program over a period of 2 school years (21 months) can result in enhancement in muscle strength with no detrimental effect on growth.