An 18-year-old man developed consecutive homonymous hemianopias that were eventually attributed to mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS). The diagnosis was initially suspected when brain CT scans showed bilateral dystrophic basal ganglia calcifications and MR spectroscopy later showed a prominent lactate peak. Diffusion-weighted MRI showed progressive evolution of restricted proton diffusion at the margins of the lesion from day 3 through 3 weeks. Genetic testing from peripheral blood confirmed an A3243G transition in the patient's MTTL1 gene encoding the transfer RNA for leucine. The patient's visual function improved, but severe atrophy of gray and white matter was visible on MRI.