Diagnosis of mitochondrial disease.<h4>Diagnoses</h4>Mitochondrial disease with GTPBP3 gene mutations.<h4>Outcomes</h4>After receiving drug treatment, the patient exhibited a reduction in lactate levels, an enhanced physical condition compared to prior assessments, and demonstrated average intellectual development.<h4>Lessons subsections</h4>For echocardiographic indications of LV wall thickening and LV enlargement, one needs to be alert to the possibility of hereditary cardiomyopathy, especially in children. The gene discussed is GTPBP3; the disease is mitochondrial disease.