Auditory evoked magnetic fields were recorded from 15 patients with acute cerebral infarction and 11 healthy volunteers using magnetoencephalography. The auditory stimuli of 2 kHz pure tone were binaurally presented with an interstimulus interval of 1 second. The intensity of stimuli was 90 dB and the stimulus duration was 8 ms. The results showed that the M100 was the prominent response, peaking approximately 100 ms after stimulus onset in all subjects. It originated from the area close to Heschl's gyrus. In the patient group, the peak latency of M100 responses was significantly prolonged, and the mean strength of equivalent current dipole was significantly smaller in the affected hemisphere. The three-dimensional inter-hemispheric difference of the M100 positions was increased in the patient group. Our experimental findings suggested that impairment of cerebral function in patients with acute ischemic stroke can be detected using magnetoencephalography with the higher spatial resolution and temporal resolution. Magnetoencephalography could provide objective and sensitive indices to estimate auditory cortex function in patients with acute cerebral infarction.