Adults undergoing hematopoietic stem cell transplant (HSCT) are at risk for vitamin D deficiency. After HSCT, exposure to sunlight is restricted, and patients may experience poor nutrition and malabsorption from HSCT-related side effects. Vitamin D affects bone health and immunologic processes. The aim of this project is to establish a process for monitoring and treating vitamin D deficiency and to evaluate if therapeutic vitamin D levels are attainable posttransplant using an HSCT vitamin D replacement algorithm. A multidisciplinary group led by advanced practice providers established a workflow for monitoring and supplementing vitamin D and created an HSCT vitamin D replacement guideline. The medical records of 144 adult HSCT patients were reviewed, and the records of another 72 patients were reviewed a year later. Historical baseline data before the intervention found that 81% of patients were vitamin D deficient and 30% received supplementation. Postintervention and at 1-year follow-up, 76% and 65% of patients were vitamin D deficient before transplant and 97.1% and 100%, respectively, received supplementation for vitamin D deficiency. Post-HSCT compliance with monitoring demonstrated that approximately 91% of patients had a vitamin D level checked within 6 months of transplant. After implementation of the algorithm, there was a statistically significant difference (p < .001) between deficient vitamin D levels pretransplant (72.9%) and posttransplant (26.4%). Results demonstrate sustained compliance over a 2-year period with monitoring and supplementation of vitamin D pre- and peritransplant. Aggressive vitamin D repletion posttransplant decreased the incidence of vitamin D deficiency in HSCT patients. Further study is needed to investigate the long-term effects of vitamin D repletion on posttransplant complications.