We evaluated the feasibility of using upper extremity anthropometrics to monitor the clinical status of 18 patients with amyotrophic lateral sclerosis (ALS). The bone-free arm muscle area (AMA) was computed using measurement of triceps skinfold thickness and the mid-upper arm circumference according to published formulae. The AMA correlated significantly with body mass, isokinetic muscle force generation, cross-sectional muscle area on computerized tomography scanning, and pulmonary functions including forced vital capacity and maximal voluntary ventilation. Serial determinations of AMA demonstrated a decline in 10 of 13 patients over 6 months. We pilot tested the use of AMA in a clinical trial of ciliary neurotrophic factor (CNTF) in the treatment of ALS. The AMA progressively decreased by 13%, 15%, and 30% in ALS patients treated with 0 microg CNTF/kg, 15 microg CNTF/kg, and 30/microg CNTF/kg, respectively, over a 9-month treatment period. We conclude that measurement of AMA provides a simple, inexpensive method to monitor the progression of muscle atrophy in ALS patients. The technique does not require effort on the part of the patient and as such, appears to have potential utility as an outcome measure in clinical drug trials.