Methemoglobinemia is a rare congenital or acquired disease of increased blood methemoglobin concentration. We documented 2 cases of children suffering from neuroblastoma whose postchemotherapy anemia, leucopenia, and stomatitis were complicated by methemoglobinemia after using a formulary oral gel (7.5% benzocaine, doxycycline, nystatin, glycerin). The complication resulted in hospital treatment. Percutaneous oxygen saturation remained at 85% and 87% despite administration of 100% oxygen through a nonrebreather mask. Arterial blood gas analysis showed an oxygen saturation of 98% and 97%, respectively. Spectroscopic measurement showed methemoglobin concentration of 42% and 35.5%, respectively. After red blood cell transfusion and oral ascorbic acid in case 1 and methylene blue in case 2, the patients’ condition improved. Although the benzocaine gel is not in use in several medical systems, it should be considered as a possible reason for methemoglobinemia.