Mitochondrial disorders have long been regarded as neuromuscular diseases only. In fact, owing to the ubiquitous nature of the oxidative phosphorylation, a broad spectrum of clinical features should be expected in mitochondrial disorders. Here, we present eight puzzling observations which give support to the view that a disorder of oxidative phosphorylation can give rise to any symptom in any organ or tissue with any apparent mode of inheritance. Consequently, we suggest giving consideration to the diagnosis of a mitochondrial disorder when dealing with an unexplained association of symptoms, with an early onset and a rapidly progressive course involving seemingly unrelated organs. Determination of lactate/pyruvate and ketone body molar ratios in plasma can help to select patients at risk for this condition.