Abstract Forty-eight left ventricular cineangiocardiograms of patients with ventricular septal defects investigated over a two year period were reviewed. In 16 cases aneurysms of the membranous septum were present, 14 occurring in 26 cases of small ventricular septal defect. Diagnosis was made by cineangiocardiography. Various associated lesions and the possible causes are discussed. Spontaneous closure of the defect was documented in 1 case and in another this diagnosis was made clinically. These observations, together with the high frequency of aneurysms associated with small ventricular septal defects, suggest that aneurysmal formation in the membranous septum may occur near the time of closure of the ventricular septal defect.