In a 27-year-old woman with a large hydatid cyst of the liver, an asymptomatic cardiac cyst located in the interventricular septum was discovered as well. The diagnosis was based upon echocardiography, computerized axial tomography and nuclear magnetic resonance. There was no evidence of damage to the atrioventricular conduction system or to the cardiac valves. At operation, however, the left posterior papillary muscle and chordae were firmly adherent to the cyst. Patch closure of the resulting ventricular septal defect, and mitral valve and chordae repair were necessary. The patient is alive and well 6 months after the operation, with mild residual mitral regurgitation. Precise anatomical delineation of the hydatid cyst localization within the heart, particularly in the interventricular septum, and its relations with the various cardiac structures is a difficult task, in spite of all the imaging techniques available. The surgical approach therefore has to be very careful and mindful of the potential complications.