We report on a preterm Japanese male baby with Leigh syndrome, i.e., intrauterine growth restriction, central apnea, and feeding difficulty. These signs improved at 41 weeks of corrected age. At that time, brain magnetic resonance imaging revealed increased signal in diffusion-weighted imaging in the parietal white matter, bilaterally and symmetrically not respecting vascular territory or boundaries. However, clinical improvement deterred us from further investigation. About 3 months later, he manifested frequent ictal apnea with myoclonic seizures and deterioration of consciousness to semicoma. Subsequent diffusion-weighted imaging revealed increased signal in the bilateral symmetric thalamus, internal segments of the globus pallidus, substantia nigra, and pontine tegmentum. Laboratory investigation indicated remarkable elevation of lactate levels in cerebrospinal fluid. The diagnosis was of Leigh syndrome. We think this is the first reported case of Leigh encephalopathy with transient abnormality of diffusion-weighted imaging of the white matter before apparent clinical onset. Leigh syndrome should be included in the differential diagnosis of abnormality of diffusion-weighted imaging in white matter without apparent clinical signs.