The effect of protein supplementation on athletic performance and hormonal changes was examined in 21 experienced collegiate strength/power athletes participating in a 12-week resistance training program. Subjects were randomly assigned to either a protein supplement (PR; n = 11) or a placebo (PL; n = 10) group. During each testing session subjects were assessed for strength (one repetition maximum [1-RM] bench press and squat), power (Wingate anaerobic power test) and body composition. Resting blood samples were analyzed at weeks 0 (PRE), 6 (MID) and 12 (POST) for total testosterone, cortisol, growth hormone, and IGF-1. No difference was seen in energy intake between PR and PL (3034 ± 209 kcal and 3130 ± 266 kcal, respectively), but a significant difference in daily protein intake was seen between PR (2.00 g·kg body mass[BM]^-1·d^-1) and PL (1.24 g·kgBM^-1·d^-1). A greater change (p < 0.05) in the ∆ 1-RM squat was seen in PR (23.5 ± 13.6 kg) compared to PL (9.1 ± 11.9 kg). No other significant strength or power differences were seen between the groups. Cortisol concentrations were significantly lower at MID for PL and this difference was significantly different than PR. No significant changes were noted in resting growth hormone or IGF-1 concentrations in either group. Although protein supplementation appeared to augment lower body strength development, similar upper body strength, anaerobic power and lean tissue changes do not provide clear evidence supporting the efficacy of a 12-week protein supplementation period in experienced resistance trained athletes.