Background: HDCT/ASCT represents a curative salvage treatment for patients with GCT but is rarely used for other solid tumors. Patients undergoing HDCT/ASCT for hematologic neoplasms require revaccination for their childhood immunizations. Whether this is necessary in patients with GCT is unknown. Methods: In this prospective longitudinal study, patients with GCT undergoing HDCT-ASCT from 11/2010 to 5/2018 had serologies for Measles, Mumps, Rubella, Diphtheria, Tetanus, Polio, and Varicella Zoster measured before HDCT and at 3, 6, and in a subset, 12+ months after the last HDCT with results at these timepoints compared using descriptive statistics. In addition, titer levels at ≥6 months post-transplant were matched 1:1 for age and gender with HL patients who underwent HDCT/ASCT during the same time period. Immunity was compared between cohorts using the Cochran-Mantel-Haenszel test. Results: Of 80 patients with GCT (median age 30, 84% nonseminoma), 91% received 3 sequential transplants and 68 had repeat titers at ≥6 months. Immunity at baseline was >95% for Diphtheria, Tetanus and Polio and 89% for Varicella Zoster but lower for Measles (74%), Mumps (85%), and Rubella (83%) (Table). Compared to baseline, proportional immunity for all infections was similar at 3, 6, and 12 months post-transplant in the GCT population (≥6 months shown in Table). Matching resulted in 58 GCT-HL pairs. One-year immunity was numerically lower for most infections in the HL vs. GCT patients and significantly decreased for Measles and Rubella (Table). Conclusions: To our knowledge, this is the first study to assess vaccine titers following HDCT/ASCT for GCT. We demonstrate that HDCT/ASCT does not result in loss of immunity to childhood vaccines and that GCT patients retain protective titers more frequently than those with HL. However, 15-31% of GCT patients lack MMR immunity at baseline and at 1-year post-ASCT. Therefore, we recommend checking MMR titers at 1-year post-ASCT with revaccination of those lacking immunity. Titer evaluation and revaccination is not necessary for other childhood immunizations.