AbstractQuantitative electromyography (EMG) was performed in 223 infantile and juvenile cases of spinal muscular atrophy (SMA), which were classified into 3 groups: (A) form la, Werdnig‐Hoffmann disease; (B) forms lb and II, intermediate forms; and (C) form III, Kugelberg‐Welander disease. The groups differed in the occurrence of spontaneous activity; only groups A and B showed spontaneous rhythmic firing of motor units, whereas in long‐standing cases, pseudomyotonic volleys appeared. The parameters of individual motor unit potentials (MUPs) differed in the different forms of the disease. Group A showed, in addition to long potentials of high amplitude, some short and low amplitude potentials, and the histograms of amplitudes and durations were bimodal. In the long‐standing cases, the values of these parameters were shifted to longer durations and higher amplitudes of motor unit potentials. However, in long‐standing cases of the benign group C, the short, low potentials appeared as well as socalled linked potentials. In the very early stage of the disease, the children who were found to be suffering from chronic forms of SMA—both malignant (form lb and II or benign group C)—had an EMG record that was slightly different from that of acute form la. Their EMG record shows more so‐called “spinal” signs, particularly in the benign group C (Kugelberg‐Welander disease). These increasing features of chronic anterior horn cell involvement followed a pattern of reinnervation and hypertrophy of muscle fibers. These phenomena were particularly seen in the benign group C. These findings indicate that in the early stage of SMA, the EMG not only has diagnostic, but a