A rating scale has been developed to provide a quantitative estimate of clinical status and disease progression in amyotrophic lateral sclerosis (ALS). This scale includes assessment of swallowing, speech, and respiratory function, and both strength and function of upper and lower extremity musculature. The evaluation is relatively simple to perform and yields reproducible data for both a total ALS score and a score for each group of functions tested. A score of 30 points is normal; 164 points indicates maximal dysfunction. The total ALS score increased in a linear fashion in each of 74 patients followed for at least one year. Among patients, the rate of disease progression varied twenty-fold, with a continuous distribution from the slowest to most rapid course. Thirty-four percent of patients exhibited a rapid change of greater than 48 points in the year, predicting progression to a terminal stage in less than two years; 19% of patients exhibited a slow change of less than 13 points in one year, predicting progression to a terminal stage over at least five years. This ALS scoring system should permit more accurate assessment of drug efficacy in clinical trials and correlation of rates of progression with clinical variables.