In view of the conflicting results about the links between lower and upper motor neuron (LMN, UMN) dysfunction in amyotrophic lateral sclerosis (ALS), we undertook this study to correlate their changes over time. Single motor units (MUs) were characterized by their macro-MU potentials, twitch amplitude, and excitatory responses to transcranial magnetic stimulation (TMS). Ten ALS patients were studied 2 to 4 times and their data were subdivided into epochs corresponding to mean disease duration of 12 (58 MUs), 20 (60 MUs), 32 (50 MUs), 43 (40 MUs), and 168 months (55 MUs). The MU size increased and the contractile effectiveness and the excitatory response rates decreased significantly with time. The contractile effectiveness of MUs producing normal excitatory responses decreased with time, whereas a gradual loss of excitatory responses was observed among MUs with normal electromechanical properties. Since no correlation was found between UMN and LMN dysfunction, we conclude that UMN and LMN probably degenerate independently in ALS.