Abstract: Two siblings with psychomotor retardation, congenital hypotonia, spasticity, and no speech acquisition underwent MRI and 99mTc ethyl cysteinate dimer SPECT imaging. The SPECT images showed a reduction in regional cerebral blood flow in the bilateral frontal cortex and cerebellum in both cases. T2-weighted and fluid attenuated inversion recovery images obtained using MRI showed delayed myelination and cortical atrophy in mainly the frontal lobes. Based on the MRI findings, the abnormal serum levels of thyroid hormone, and the gene mutation, the siblings were diagnosed as having monocarboxylate transporter 8 deficiency. A reduction in regional cerebral blood flow, as observed using SPECT, may be a common feature of monocarboxylate transporter 8 deficiency.