Introduction: Many children and adolescents who were vaccinated prior to cancer treatment lose humoral immunity after completion of therapy. Pediatricians and pediatric oncologists often recommend re-immunization, although there is little consensus on timing and approach to serologic testing. However, vaccine hesitancy in the U.S. is a growing problem. It is not known whether parents who initially permitted vaccination might demonstrate secondary hesitancy regarding re-immunization. Methods: We conducted a qualitative study to explore parental attitudes toward re-immunization after completion of cancer therapy. Twenty primary caregivers of current pediatric cancer patients participated in structured interviews exploring knowledge and understanding of immunity and vaccination; previous experiences with vaccines; and attitudes toward vaccines and revaccination. Results: Of those interviewed, 80% were female and 90% were White Non-Hispanic. Of interviewees’ children with cancer, 60% were male, 75% had been diagnosed within the past 6 months, and 45% had leukemia or lymphoma. All caregivers demonstrated a basic understanding of vaccination, but only 65% understood that it was possible to lose immunity even with previous vaccination. All caregivers were willing to have their children immunized if tests showed lack of humoral immunity, with 85% expressing a preference for testing prior to revaccination. Conclusions: Primary caregivers of children with cancer are willing to consider re-immunization interest but do express some secondary hesitancy and strongly prefer that the need for re-immunization be established via serologic testing, rather than performed empirically. Caregivers’ beliefs and preferences regarding re-immunization in pediatric oncology should be considered in the development of post-treatment guidelines.