Introduction: Inhalation aromatherapy may help in controlling nausea, vomiting, and anxiety during autologous hematopoietic stem cell transplantation (AHSCT) by masking the malodor of dimethyl sulfoxide. This study examined the preventative effects of inhalation aromatherapy with orange essential oil on severity of nausea, vomiting episodes, and anxiety during AHSCT. Methods: This open-label randomized controlled trial was conducted at two stem cell transplantation units in Ankara, Turkey. A total of 70 patients were randomly assigned to either an intervention (n = 35) or a control group (n = 35). Patients in the intervention group inhaled the odor from six drops of pure orange essential oil during AHSCT. Patients in the control group received routine care. Nausea severity was evaluated at baseline and the beginning of each new infusion bag. Number of vomiting and retching episodes was recorded during the AHSCT. Anxiety level was measured just after the completion of AHSCT. Results: The nausea severity (p = 0.931; 0.721 and 0.363 for the first, second and third bags, respectively), and the number of retching episodes (p = 0.741 and 0.485 for the first and second bags, respectively) and vomiting episodes (p = 0.396 and 0.957 for the first and second bags, respectively) showed no significant differences between the groups. The mean anxiety score was significantly lower for the intervention group than for the control group (p = 0.007). Conclusion: Inhalation aromatherapy with orange essential oil may be useful to alleviate anxiety during AHSCT; however, it does not appear to decrease nausea severity, and number of vomiting and retching episodes. Further studies are needed to confirm the study findings.