Pulmonary hypertension (PH) is an infrequently reported complication after hematopoietic stem cell transplantation, and its etiology and therapeutic strategies, especially in infants, remain unclear. We report a case of severe PH that developed in an infant with acute leukemia following administration of busulfan as a preconditioner for cord blood transplantation; the case was successfully treated with sildenafil and beraprost, which to our knowledge is the first reported successful use of this regimen in PH following transplantation for infantile leukemia. From a review of all previous reports, use of busulfan in infants may raise the risk of developing PH, and unlike definitive pulmonary veno-occlusive disease, PH in this subgroup may be reversible by early detection and treatment.