Defects in the muscular portion of the interventricular septum are frequently multiple. Although their incidence is significant, scant attention has been paid them as a distinct entity. This report concerns seven patients with multiple muscular ventricular septal defects seen between 1956 and 1962. The patients ranged in age from 4 months to 9 years; five were less than 2 years old. Symptoms and signs of growth retardation, failure to thrive, and marked cardiac decompensation were present in each case. Electrocardiographic evidence of combined ventricular hypertrophy and incomplete right bundle-branch block was noted in each instance. Right heart catheterization demonstrated marked pulmonary hypertension of the high-flow, low-resistance type. These routine studies could not distinguish multiple muscular ventricular septal defects from the more usual membranous septal defect. All patients had cardiac surgery. There was one survivor. In three instances difficulty in identifying all of the multiple fenestrations prevented complete surgical repair. Postmortem examination was done in six patients. The sievelike appearance of the muscular ventricular septum was the most striking gross finding in the majority of the cases. Histologic analysis of the pulmonary vascular structural alterations did not reveal marked changes in any of the cases.In contrast to the widespread belief that most isolated muscular septal defects are innocuous, these seven examples of multiple muscular defects were associated with a picture of severe heart disease. Every effort should be made to diagnose the lesion prior to surgery. Recognition of the multiple nature of the defect is a necessary preliminary to successful therapy. It is hoped that preoperative left heart angiocardiography will successfully demonstrate the multiple lesions.