In order to analyze the EMG dynamics in acute and chronic polymyositis 44 patients were examined. Thirty-four were seen in the acute stage, 28 in the chronic stage and 18 serially. Investigations included quantitative electromyography using the Polish minicomputer "ANOPS 105" connected to a DISA electromyograph. Additionally fiber density was analyzed by single fiber electromyography in the chronic stage only. The acute stage findings confirmed the observations of earlier authors with the classical expression of excessive spontaneous activity, polyphasic potentials of short duration and low amplitude. In the chronic stage, motor unit potentials with increased duration and amplitude and with late components of the type seen in satellite potentials were noted. This was compared with the increased fiber density found at this stage. Additionally, in some muscles in the chronic stage, motor unit potentials were seen with increased duration, but also a reduction in the mean amplitude of motor unit potentials counted by the automatic analysis method. The decreased amplitude of the motor unit potentials in the chronic polymyositis may be the result of the smaller size of regenerating muscle fibers.