While psychosocial factors are known to affect cancer progression via biobehavioral pathways in many patient populations, these relationships remain largely unexplored in hematopoietic stem cell transplant (HCT) patients. The purpose of this paper is to critically review the literature regarding psychosocial and endocrine/immune aspects of HCT, with an emphasis on exploring pathways that may mediate the associations between psychosocial factors and disease outcomes. These include the roles of catecholamines, glucocorticoids, inflammation, vascular endothelial growth factor (VEGF), immune reconstitution and infectious susceptibility, as well as the new opportunities available in genomics research. We also discuss the implications for potential immunomodulating psychosocial interventions. Elucidating the biological pathways that account for the associations between psychosocial factors and clinical course could ultimately lead to improved outcomes for this psychologically and immunologically vulnerable population.