Background Leukemia and hematopoietic stem cell transplantation (HSCT) create physical, psychological, social, and spiritual distresses in patients. Understanding this threatening situation in adults with leukemia undergoing HSCT will assist health care professionals in providing holistic care to the patients. Objectives The aim of the present study was exploring the perceived threat in adults with leukemia undergoing HSCT. Patients and Methods This article is part of a longitudinal qualitative study which used the grounded theory approach and was conducted in 2009-2011. Ten adults with acute leukemia scheduled for HSCT were recruited from the Hematology–Oncology Research Center and Stem Cell Transplantation, Shariati Hospital in Tehran, Iran. A series of pre-transplant and post-transplant in-depth interviews were held in the hospital’s HSCT wards. Totally, 18 interviews were conducted. Three written narratives were also obtained from the participants. The Corbin and Strauss approach was used to analyze the data. Results Perceived threat was one of the main categories that emerged from the data. This category included four subcategories, "inattention to the signs and symptoms", "doubt and anxiety", "perception of danger and time limitation" and "change of life conditions", which occurred in linear progression over time. Conclusion Suffering from leukemia and experiencing HSCT are events that are uniquely perceived by patients. This threatening situation can significantly effect perception of patients and cause temporary or permanent alterations in patients' lives. Health care professionals can help these patients by deeper understanding of their experiences and effective interventions.