A novel leukoencephalopathy, termed hypomyelinating leukoencephalopathy affecting early myelinating structures (HEMS), was recently described. Here we report on two patients affected by HEMS with a clinical picture characterized by early-onset nystagmus and thereafter progressive cerebellar signs and mild spasticity predominantly affecting the legs. In one patient, who has the longest follow-up described to date, we detected a mild worsening of the clinical and neuroradiologic picture after a long period of stability lasting until age 6 years. The most recent magnetic resonance image, performed at the age of 11 years, showed a more severe neuroradiologic picture characterized by involvement of almost the entire supratentorial white matter, with relative sparing of the subcortical fibers. We also provide spectroscopy results, not previously reported in this disorder, that support the idea of a progressive disease course on neuroimaging. Our findings suggest that HEMS patients should undergo a new magnetic resonance imaging evaluation after a certain interval to look for possible progression of the abnormalities. © Georg Thieme Verlag KG Stuttgart. New York.