Morphological findings contributing to a failed Fontan procedure. Twelve-year experience. A group of 37 patients (age range, 3 months to 29 years) who died after the modified Fontan procedure (within 2 months), representing 15% of the 245 patients undergoing this procedure from 1976 through 1988, was reviewed to determine the causes of death. The three morphological groups were 1) univentricular atrioventricular connection (n = 19), 2) tricuspid atresia (n = 9), and 3) other complex malformations (n = 9). Subaortic stenosis was present in 15 patients (40%) in this group compared with 31 of 208 early survivors (15%) (p = 0.008). Pulmonary artery banding, identified as a risk factor in our previous experience, was performed in 14 patients, nine of whom had subaortic stenosis. Nine palliative procedures for subaortic stenosis were performed in eight patients--before the Fontan procedure in three patients and concurrently with the Fontan procedure in five patients. Myocardial hypertrophy and signs of acute ischemic injury were common findings at autopsy (n = 25) and were particularly prominent in all patients with univentricular heart of left ventricular morphology associated with subaortic stenosis and previous pulmonary artery banding (n = 7). We conclude that the present majority of deaths occurring after the modified Fontan procedure are myocardial in nature and attributable to advanced myocardial hypertrophy that is potentiated by previous pulmonary artery banding and subaortic stenosis. 