It is well known that there is sexual dimorphism with regards to body composition. Healthy adult women have less lean body and muscle mass and more fat than men and the age-associated decrease in muscle mass is slower in women than in men. Several studies, however, indicate that there is no difference in the basal rate of muscle protein synthesis or muscle protein breakdown or the anabolic responses to nutritional stimuli and resistance exercise in young and middle-aged adult men and women. Nevertheless, recent evidence suggests that aging affects muscle protein turnover differently in men and women. The basal rate of muscle protein synthesis is greater in old women than in old men; in addition, both old women and old men are resistant to the anabolic effects of exercise and nutrition and old women appear to be more resistant than old men. This suggests that differences in muscle protein turnover between men and women might be most apparent when muscle mass is changing (i.e., during aging vs. young and middle-adulthood when muscle mass is steady). Men and women therefore need to be considered separately when evaluating muscle protein synthesis rates in older adults.