Impaired mitochondrial energy production probably plays a role in motor neuron death in amyotrophic lateral sclerosis (ALS) and has been found not only in motor neurons but also in skeletal muscle of patients with ALS. 31P magnetic resonance spectroscopy (31P-MRS) has the potential to reflect the energy metabolism of skeletal muscle in vivo. We sought to determine whether an altered mitochondrial energy metabolism of the muscle cell in patients with SALS can be detected by 31P-MRS, and to this end we recorded 31P-MR spectra of the gastrocnemius muscle of patients with ALS under a standardized isometric muscle exercise protocol. In a prospective setting we compared ten patients with sporadic ALS and 38 age-matched controls. The patients were characterized by a disease duration of approximate 18 months and classified as having probable to definite ALS by means of the revised El Escorial criteria. The time constant of oxidative PCr recovery after aerobic (tau1) and ischaemic muscle contraction (tau2) was used to determine the capacity of mitochondrial ATP formation. We found that mitochondrial impairment in skeletal muscle of patients with ALS could not be confirmed by 31P-MRS and therefore cannot be used as a surrogate factor of the disease.