A current hypothesis claims that an increase of blood flow is required for oxygen consumption to rise during neuronal excitation (activation). Chronic progressive external ophthalmoplegia is a mitochondrial disease associated with deletions of mtDNA or by point mutation of tRNA genes. We tested the hypothesis that the cerebral metabolic rate of oxygen (CMRO2) may not rise in this disorder if the accompanying cerebral blood flow increase is insufficient. Two patients with progressive external ophthalmoplegia were visually stimulated with a colored checkerboard pattern reversing as different frequencies. When stimulated, Patient 1 had a small increase of cerebral blood flow, while Patient 2 had no cerebral blood flow increase. In the visually active state, the patients had no significant change of CMRO2, while healthy subjects had a pronounced increase of CMRO2 in the pericalcarine visual cortex at 4 Hz and a further slight increase at 8 Hz during activation.