Objective: Mitochondrial diseases are clinically heterogenous group of disorders with widely varying clinical features. Diagnosis can be difficult and requires synthesis of clinical, biochemical, histopathological and molecular data. These investigations may not be available in most medical centers. Muscle biopsy provides an important information to confirm a mitochondrial disease. Ragged red fibers and cytochrome oxidase negative fibers are the morphological hallmark of mitochondrial diseases. In this study, we aimed to analyze clinical features and histopathological findings of patients with mitochondrial abnormalities on muscle biopsy.