The patient was a 4-month-old infant, who underwent persistent ductus arteriosus interruption with titanium clips at the age of 13 days and, since the age of 2 months, had crises of hypoxia and hypertonicity. After clinical investigation, the presence of pulmonary hypertension was confirmed and left ventricular inflow tract obstruction was suspected. The patient underwent surgical treatment at the age of 4 months, during which right and left ventricular endocardial fibrosis was identified. The fibrosis was resected, but the infant had an unfavorable clinical evolution with significant diastolic restriction and died on the sixth postoperative day. Anatomicopathological and surgical findings suggested endomyocardial fibrosis, although that pathology is very rare at the patient's age.