A. Specific Aims The authors propose to study the change in muscle mass over time in patients with ALS. Muscle atrophy is a very common feature of the disease, and in clinical experience correlates with progressing weakness. As such, it is a qualitative marker of disease progress, although this lacks a quantitative marker. Muscle ultrasound is a safe, noninvasive, and rapid method of measuring muscle volume or thickness (1). Recently Arts, et al published an assessment of muscle ultrasound, specifically including reduced muscle depth and increased ultrasound echogenicity (2). We believe determining quantitatively the rate of progression of muscle mass in a group of patients with ALS will lead to a clinically useful tool to serve as a marker of disease progression, which may be useful in clinical trials for new therapies for this disease.