Macroelectromyography (macroEMG) is a technique that provides information about the size of a motor unit which the conventional EMG is not able to. We used macroEMG to study 11 cases of motor neuron disease (MND), presenting with upper extremities symptoms (no lower extremities symptoms). Muscles we selected to study were the biceps brachii and the tibialis anterior. Amplitude, area and fiber density (FD) were analysed. The controlled groups consisted of 11 cases of myasthenia gravis (MG) and 11 normal subjects. Our study demonstrated that in the MND group, amplitude, area and FD of both muscles were significantly increased over those of the normal subjects. We also found that the amplitude and area of biceps brachii in patients who had clinical symptoms less than 2 years, were significantly higher than those over 2 years. However, there were no significant differences between the two groups of the tibialis anterior. This was probably due to the fact that the disease process was not severe enough to cause significant differences. Further follow up studies are necessary to clarify this question. In MG, amplitude, area and FD were not significantly different than those of the normal subjects. Increased amplitude, area and FD in MND provides information regarding the reinnervation process of the disease. As the disease progresses, the above findings may decrease because the motor neurons can no longer support the enlarged motor unit. All of the MND patients in our study, had no symptoms in the lower extremities; however, we found there were significant changes in the macro-EMG findings of the tibialis anterior. This indicates that macro-EMG can detect early electrophysiological changes in the motor unit before clinical symptoms develop.