Studies of the effects of dietary protein level on human metabolism have usually concentrated on the effects of protein deprivation and on establishing a minimum dietary requirement. By contrast, less is known about the effects of very high protein diets, although general levels of protein intake in the developed world are increasing, and high protein diets have been advocated for maintaining or increasing muscle mass in certain groups of the population. This article, therefore, examines the response of protein metabolism to high dietary protein, studied in adults by nitrogen balance and isotopic tracer techniques, and concentrating on the evidence for increased lean body mass. It is concluded that high protein feeding initially results in protein retention, with greater cycling of body protein in response to meals, but that neither N-balance nor isotopic tracer methods possess sufficient sensitivity to detect whether a long term increase in functional lean tissue ensues. Improved methods of body composition measurement will be needed to establish this. Moreover, the absence of strong evidence that high protein diets confer any advantage in terms of strength or health must be weighed against potentially injurious consequences.