Auditory stimuli elicit auditory evoked magnetic fields (AEFs) called P50m and N100m, which index preconscious auditory processing in human. We investigated with a whole-head magnetometer whether Parkinson's disease (PD) impairs parallel preattentive auditory processing between the hemispheres. Stimulus blocks consisting of standard (80%) and deviant (20%) tones were monaurally presented in a passive condition to 11 PD patients with unilateral motor symptoms and to 11 age-matched healthy controls. The constant interstimulus intervals (ISIs) were 0.5 s and 2.5 s in separate blocks. The interhemispheric latency differences of the P50m and N100m were significantly lengthened in PD patients in the left-ear condition. This might be caused by the basal ganglia dysfunction in PD together with the simultaneous age-related neural degeneration predominant in the left auditory cortex.