We have studied motor, somatosensory, brainstem auditory and visual evoked potentials in two patients with ML III without overt neurologic symptoms. Both our patients showed abnormal central motor functions upon magnetic cortical stimulation; in addition, in one of them, we observed a decreased conduction velocity of the nervous impulse in the central tract of the upper limb somatosensory pathway, from the cervical spinal cord to sensory cortex. Our results suggest that central motor pathways are affected in both children and also that the central somatosensory system is involved in one of them. We propose that the electrophysiological techniques might be of help to detect, locate and monitor minimal neurological lesions of mild lysosomal disorders like ML III.