PURPOSE: Investigation of the clinical, imaging, and in vivo MR spectroscopy (MRS) characteristics of disorders of mitochondrial function. METHODS: Clinical, imaging (five CT and 20 MR examinations), and MRS (six studies in five patients) findings in 19 patients with mitochondrial disorders were retrospectively reviewed. Results were critically analyzed and, when applicable, compared with results in the literature. RESULTS: Patients included four with mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS), two with myoclonus, epilepsy, and ragged red fibers (MERRF), two with Kearns-Sayre syndrome, seven with Leigh syndrome, one with progressive cerebral poliodystrophy (Alpers syndrome), and three with trichopoliodystrophy (Menkes disease). MELAS, MERRF, and Kearns-Sayre tended to occur in older children and adults, whereas Leigh syndrome, Alpers syndrome, and Menkes disease occurred in infants and young children. All diseases involved gray matter early in their course, manifest primarily as T2 prolongation, with the deep cerebral nuclei being involved more often than the cerebral cortex. When T2 prolongation was seen in the white matter (MELAS, MERRF, Kearns-Sayre, Leigh), the peripheral and retrotrigonal white matter showed early involvement. Patients with Menkes disease showed rapidly progressive atrophy accompanied by large subdural hematomas. Proton MRS showed an elevated lactate level in involved regions of the brain; the lactate peak disappeared in old areas of T2 prolongation. CONCLUSIONS: Mitochondrial disorders have a wide range of both clinical and imaging findings. Although no one set of findings is diagnostic of these disorders, the combination of deep gray matter involvement and peripheral white matter involvement in young adults or children should suggest the diagnosis, especially when associated with an elevated lactate level on proton MRS.