We report two children who developed severe, fatal pulmonary hypertension (PHT) after mismatched unrelated donor cord blood transplantation using reduced intensity conditioning for HLH. PHT was diagnosed on post mortem lung biopsies with no evidence of HLH, pulmonary veno-occlusive disease, infection or of idiopathic pulmonary hypertension. PHT may be an association with HLH and physicians treating HLH should be aware of this potential association. Pediatr Blood Cancer 2013; 60: 521–523. © 2012 Wiley Periodicals, Inc.