Evidence is limited but suggests that overweight and obesity may attenuate the muscle strength response to resistance training (RT). PURPOSE To investigate the influence of overweight and obesity on the muscle strength and size adaptations to a 12 wk unilateral, upper arm RT program in a large sample of apparently healthy, young adults. METHODS Subjects were 449 normal weight (NW) (mean ± SEM) (22.0 ± 0.1 kg/m2, 23.3 ± 0.2 yr) and 238 overweight to obese (OW) (29.2 ± 0.3 kg/m2, 25.6 ± 0.4 yr) men (n=285) and women (n=402) who had not performed RT for 1 yr prior to study participation. Subjects underwent 12 wks (2 d/wk) of supervised RT on the non-dominant arm. Peak elbow flexor muscle strength (kg) was evaluated by isometric testing and 1 repetition maximum (1RM). Biceps muscle cross sectional area (cm2) was measured by magnetic resonance imaging. Muscle strength data were analyzed with no correction (absolute), corrected for baseline strength values (relative), and scaled for body weight by the allometric method (allometric). Biceps cross sectional area was analyzed with no correction (absolute) and corrected for initial values (relative). Multivariate ANOVA was used to detect if muscle strength and size differed between NW and OW pre-RT, post-RT, and pre- to post-RT. RESULTS OW had greater pre-RT absolute elbow flexor peak isometric (50.4 ± 1.0 vs 42.2 ± 0.7 kg) and 1RM (10.1 ± 0.2 vs 8.3 ± 0.1 kg) strength and biceps size (19.5 ± 0.3 vs 16.4 ± 0.2 cm2) than NW (p <0.001). Similarly, OW had greater post-RT absolute elbow flexor peak isometric (58.1 ± 1.1 vs 50.6 ± 0.8 kg) and 1RM (14.2 ± 0.2 vs 12.4 ± 0.1 kg) strength and biceps size (23.1 ± 0.4 vs 19.6 ± 0.3 cm2) than NW (p <0.001). Absolute peak elbow flexor strength gains were similar between OW and NW pre- to post-RT. In contrast, NW had greater relative peak elbow flexor isometric (21.2 ± 1.0 vs 17.4 ± 1.4%) and 1RM (54.3 ± 1.5 vs 49.0 ± 2.0%) strength increases than OW pre- to post-RT (p <0.05). Further, NW had greater allometric peak elbow flexor isometric (0.48 ± 0.02 kg·kg−0.67 vs 0.40 ± 0.03 kg·kg−0.67) and 1 RM (0.25 ± 0.00 vs 0.22 ± 0.01 kg·kg−0.67) strength increases than OW pre- to post-RT (p <0.05). Absolute biceps size gains were greater in OW than NW (p <0.001); however, relative biceps size increases were similar between OW and NW (18.4 ± 0.7 vs 19.4 ± 0.5%) pre- to post-RT. CONCLUSION Despite similar relative increases in biceps size, NW had greater relative and allometric gains in peak elbow flexor strength than NW. Excess overall adiposity may limit the muscle strength response to RT in young adults. Supported by NIH-NINDS R01 NS40606-02