There is limited and conflicting data regarding loss of immunity in childhood cancer survivors who did not undergo hematopoietic stem cell transplantation. This retrospective study included 28 childhood cancer survivors whose treatment consisted of at least 3 months of chemotherapy. Decreased seropositivity for measles, mumps, rubella, varicella, tetanus, and hepatitis B was found in patients across all categories of malignancy compared to the general population. Results were more pronounced for those with hematological malignancies. This study indicates that pediatric cancer survivors, especially those with hematological malignancies, may have greater loss of protective antibodies from primary vaccinations.