Patients with severe aplastic anemia (SAA) and an absolute neutrophil count (ANC) of 0 typically have fatal outcomes. We defined fulminant AA (FAA) as ANC = 0 for at least 2 weeks prior to and after immunosuppressive therapy (IST). We analyzed the outcomes of 35 children with FAA among 288 children who enrolled in a prospective study for AA (AA-97 study). AA was classified as FAA (n = 35), very SAA (vSAA; n = 129), or SAA (n = 124). All of the children received the IST with horse anti-thymocyte globulin (ATG) and cyclosporine (CsA). A significantly lower response rate at 6 months was seen in children with FAA when compared to those with vSAA or SAA (40.0, 63.6, and 63.7 %, respectively; p = 0.027). Of 20 nonresponder patients in the FAA group, 11 were rescued by alternative donor transplantation, and 5 patients showed a late response after 6 months. Consequently, no significant difference was noted in overall survival when comparing the FAA, vSAA, and SAA groups (88.5, 95.8, and 96.8 %). These findings indicate that IST with ATG and CsA is justified as a first-line treatment for children with FAA who lack a human leukocyte antigen-matched sibling donor.