6558 Background: Splenomegaly is a hallmark of myelofibrosis, thought to be correlated with graft failure and worsened overall survival in transplant patients. Past studies have utilized manual spleen measurement, potentially affecting accuracy. Using 3D volume can more precisely measure spleen size. Hence, the true impact of splenomegaly on transplant outcomes remains unclear. In this retrospective study, we examined myelofibrosis patients who underwent HCT and, using 3D volumetrics, analyzed the impact of splenomegaly and volume reduction over time on post HCT outcomes. Methods: This retrospective study included 130 City of Hope myelofibrosis patients with pre HCT imaging (2004–2023). Patients were divided into pre HCT spleen volume of >3000 cc and <3000 cc, with >3000 cc considered massive splenomegaly. Spleen volume was measured using Vitrea 3D volumetric software. For each patient, depending on imaging available, volume was measured 0-3 months pre HCT, 3-6 months, 6-9 months, and 9-12 months post HCT with corresponding volume reduction calculations. Pre HCT spleen volume was also temporally correlated with several post HCT outcomes. Results: Median age at HCT was 60 (17-75), and 95% of patients received the Fludarabine/Melphalan conditioning regimen. Median spleen volume pre HCT was 1150 cc (41-6695 cc). Sixteen patients measured >3000cc, and 114 patients measured <3000cc. Median spleen volume 3-6 months post HCT was 661 cc (80-1731 cc) with a median 48% volume reduction. The data suggests that majority of volume reduction occurred during this time (table). Five-year overall survival (OS) was 72.8%. Relapse was 11.0% at 1 year and 14.7% at 5 years. NRM at 5 years was 17.3%. ANC engraftment peaked at 28 days at 92.3%. Pre HCT spleen volume (<1000 cc, 1000-2000 cc, >2000 cc) did not have a significant impact on OS (P = 0.992), disease free survival (P = 0.783), relapse (P = 0.187), NRM (P = 0.627), or ANC Engraftment (P = 0.526). There was a statistically insignificant downward trend for platelet engraftment (P = 0.162). For patients with or without massive splenomegaly (>3000cc), there was no significant effect with any of the analyzed post HCT outcomes. Conclusions: This retrospective study includes an analysis of spleen volume by 3D volumetrics, a more accurate measurement of spleen size, and whether it significantly impacts post HCT outcomes. Our data analysis demonstrates that splenomegaly, massive or not, does not significantly impact transplant outcomes for patients receiving a reduced intensity conditioning regimen.[Table: see text]