It remains unclear whether brain energetics is disturbed in patients with mitochondrial disease without clinical central nervous system involvement (MDW). The authors used the high temporal and spatial resolution phosphorus magnetic resonance spectroscopy (31P MRS) technique that they developed to study high energy phosphates (HEPs) and intracellular pH (pH) in the visual cortex of 9 normal subjects and 5 MDW patients with single mtDNA deletion at rest, during, and after visual activation. In normal subjects, HEPs remained unchanged during activation but rose significantly (by 17%) during recovery, and pH increased during visual activation with a slow return to rest values. In MDW patients, HEPs were within the normal range at rest and did not change during activation, but fell significantly (by 22%) in the recovery period; pH did not reveal a homogeneous pattern. In the brain of patients with MDW, energy balance remains normal until oxidative metabolism is intensively stressed, as during a postactivation phase. The heterogeneity of the physicochemical environment (that is, pH) suggests various degrees of subclinical brain involvement. The combined use of MRS and brain activation is fundamental for the study of brain energetics and may prove an important diagnostic tool in patients with MDW.