Hydatid cysts of the heart are very rare in child. In this report we describe an interesting and unique case of an 8-year-old boy with a large cardiac intracavitary hydatid cyst filling the left ventricle. Echocardiography, computerized tomography, magnetic resonance imaging and serologic test were necessary for the diagnosis. Once assessing the diagnosis, an emergency open heart surgery was necessary to prevent the complications. Surgery associated to medical treatment provides good results as demonstrated in this case report.