In this study the H-reflex and M-wave were evoked in a group of ALS patients, to correlate the findings with the clinical state, and to investigate whether a statistical approach for assessing H-reflex changes in the presence of a constant M-wave could be reproducible and helpful in monitoring the course of amyotrophic lateral sclerosis (ALS). The H-reflex and M-wave from the soleus muscle were evoked at different stimulus strengths in 35 patients with definite ALS during the course of their illness. The mean amplitude of the H-reflexes (H-mean) obtained in different sessions within an established range of mean M-response amplitude (M-mean) was calculated. For each patient, M-mean was made constant across sessions. H-mean showed high reproducibility and two different trends of changes which emerged in a 1 year follow-up within the population: a significant progressive increase and a steady decrease. When grouped on the basis of their H-mean trend, the patients did not differ in terms of any clinical variables considered. However, the group with progressive increase of H-mean showed a better prognosis. This study has shown that H-mean is effective in assessing the clinical course of ALS and could be useful in monitoring drug effects during clinical trials.