Background: Chemotherapy suppresses the immunoglobulin production due to the cell toxicity. Decreased immunoglobulins levels results in opportunistic infections. Assessment of immunoglobulin G (IgG) serum levels against selected vaccine-preventable diseases (VPD) before and 6 months after chemotherapy (AC) in a group of Iranian cancerous children. Methods: Serum levels of Rubella, Diphtheria toxin, Hepatitis B virus (HBV), Tetanus Toxoid, Mumps and Measles IgGs of were measured among 30 previously vaccinated malignant children. Six months AC, IgG serum levels were retested to compare with the before treatment concentration. Results: 17 (56.7%) individuals were male and the age mean±SD = 7.69±3.09 (3-15 years). The most frequent malignancy was ALL (56.7%). Rubella IgG was decreased from 73.88±85.11 to 56.59±72.84 IU/mL (P< 0.05; r= 0.956; 33.4% became negative AC (NAC), Diphtheria toxin IgG was diminished from 0.683±0.454 to 0.174±0.248 IU/mL (P< 0.05; r= 0.601; 26.7% NAC); Anti-HBV IgG was reduced from 46.26±101.56 to 25.56±80.49 IU/mL (p< 0.05; r= 0.524; 60% NAC) and Anti-Tetanus Toxoid IgG was fall down from 1.031±0.582 to 0.321±0.408 IU/mL (p< 0.05; r= 0.365; 33.4% NAC). Anti-Measles and Anti-Mumps IgGs were not significantly changed (p> 0.05). Conclusion: Pediatric chemotherapy was associated with diminished IgG serum levels of most VPD. Quantitative estimations showed that: when the higher or lower IgG exists before chemotherapy, the higher and lower levels remain 6 months after treatment, respectively. Revaccination program is suggested against opportunistic infectious threats in Iranian pediatrics suffering cancer and undergoing chemotherapy. This approach enhances their survival and quality of life.