We herein report the case of a 41-year-old Japanese man with hereditary diffuse leukoencephalopathy with spheroids (HDLS) who carried the de novo K793T mutation in the colony-stimulating factor 1 receptor gene (CSF1R). He showed a gradual decline of his cognitive and mental functions over the following six months. On brain MRI, a thin corpus callosum with T2- and FLAIR-high signal intensity in the splenium was conspicuous, whereas cerebral deep and periventricular white matter lesions were mild. We propose that a diagnosis of HDLS should be considered in patients with presenile dementia presenting with corpus callosum lesions on MRI, even in cases with a lack of any apparent family history.