The N100m source of the auditory evoked magnetic fields (AEFs) is located more posterior in the left than the right hemisphere in male subjects. However, whether this asymmetry exists in female subjects is controversial. The present study, analyzed the N100m source positions of the AEFs due to monaural tone stimuli using a helmet-shaped 66-channel magnetoencephalography system (CTF Systems) in 62 right-handed normal adults (24 females and 38 males). The best fit sphere was calculated for each subject from the head shape reconstructed from three-dimensional magnetic resonance images. N100m source positions were estimated using a double-dipole model. The contralateral N100m response to the stimulated ear was measured relative to the midpoint of the bilateral auricular points. Statistical analysis used a standardized head size for each subject based on the mean value of all subjects (r = 8.27 cm). In females, the N100m dipole positions were 0.00 +/- 1.02 cm anterior in the left hemisphere and 0.35 +/- 0.74 cm in the right (p < 0.02). In males, the N100m dipole positions were -0.31 +/- 0.81 cm (mean +/- standard deviation) anterior in the left hemisphere and 0.61 +/- 0.78 cm in the right (p < 0.0001). The interhemispheric difference in the N100m positions was 0.35 +/- 0.65 in females and 0.92 +/- 0.77 in males (p < 0.005). In conclusion, the left hemispheric N100m is located more posterior to the right in both genders. However, this functional asymmetry is more evident in males, like previous findings of more evident anatomical asymmetry in males.