Mitochondrial diseases (MD) represent a heterogeneous group of multisystem disorders that affect tissues with high energetic demands such as muscle, nervous system, heart, endocrine system, etc, and are often manifested in children. They are caused either by a mutation in the maternally inherited mitochondrial genome, or by nuclear DNA mutation. Thirty-two patients (6 months to 16 years) were studied. All patients had full biochemical and haematological tests, neuroimaging studies, single-photon emission computed tomography in some patients, EEG, ECG, echocardiography. Lactate, ammonia, carnitine levels were measured. A molecular analysis of mitochondrial DNA was also performed by sequence. In 23 of the children MD was confirmed, in others, still not determined. Epilepsy was observed in nine patients, in combination with dilated cardiomyopathy in two children. Dilated cardiomyopathy was found in nine patients, in three patients in combination with stroke. Two paediatric patients had stroke as the first symptom of MD. Ataxia, myoclonus, lethargy, migraine, myopathy, ptosis, visual deterioration, mental delay were observed in some patients. High-frequency mutations found: A3243G, A11084G, related to the mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome (MELAS), also NADH 1 and 2, ATP synthase 6, cytochrome C oxidase, 16S and 12S rRNA, tRNA lysine, multiple mtDNA mutations. In our study three children with high-frequency MELAS mutation had no stroke, but epilepsy. On the contrary, in five patients with stroke no MELAS mutations were found. In conclusion, we may say that these disorders may present with a huge variety of symptoms, even if the same mutation is involved.