Summary. Wasting of skeletal muscle is a characteristic of many conditions besides those involving primary muscle disease, for example, cancer, cachexia and postoperative catabolism. There is now strong evidence from studies in animals, normal men and in patients with a variety of diseases to suggest that the muscle mass is regulated primarily by alterations in the protein synthetic rate and that changes in muscle protein degradation are largely secondary and adaptive. If this is so, attention should be focused on possible means of therapeutic intervention to increase protein synthesis rather than on ways of decreasing protein degradation.