© 2015 American Society for Nutrition. Background: Proteinmay play a role in preventingmuscle losswith aging. To our knowledge, there have been no long-termrandomized controlled trials to examine the effects of increased dietary protein intake on muscle health in community-dwelling older women. Objective: In this study, we evaluated the effects of whey protein supplementation on muscle mass and physical function in community-dwelling older Australian women. Methods: In this 2 y randomized, double-blind, placebo-controlled trial, women aged 70-80 y (mean 74.3 ± 2.7 y) were randomly assigned to either a high protein drink containing 30 g of whey protein (n = 109) or a placebo drink containing 2.1 g protein (n = 110) daily. Dual-energy X-ray absorptiometry appendicular skeletal muscle mass, upper arm and calf (38% tibia) muscle cross-sectional area, physical function including hand grip strength, lower limb muscle strength and Timed Up and Go test, and 24 h urinary nitrogen were measured at baseline, 1 y, and 2 y. Results: A total of 196 women with at least one follow-upmeasurement were included in this analysis. Baselinemean BMI was 26.7 ± 3.9 kg/m2 and protein intake was 76 ± 17 g/d (1.1 ± 0.3 g · kg body weight-1 · d-1). A mean increase in protein intake of ~20 g/d in the protein group was confirmed by the estimates from 24 h urinary nitrogen. Over the 2 y in both groups therewas a significant decrease in the upper arm (mean ± SE:25.59 ± 0.75cm2) and calf (-0.77 ± 0.11 cm2)muscle area, as well as hand grip strength (-1.30 ± 0.3 kg) (all P <0.05), but appendicular skeletalmusclemass did not change significantly. Therewere no significant effects of the protein intervention on any of the muscle mass or physical functionmeasures (all P > 0.05) at 1 and 2 y. Conclusion: This study showed that in protein-replete, healthy, ambulant, postmenopausal older women, 30 g/d of extra protein did not improve the maintenance of muscle mass or physical function despite evidence of deterioration in muscle measurements in the upper limb.