We report the results of a randomized study to elucidate whether addition of granulocyte colony-stimulating factor (G-CSF) to immunosuppressive therapy is valuable for the treatment of severe aplastic anemia (SAA) in adults. A total of 101 previously untreated patients (median age, 54 years; range, 19 to 75 years) were randomized to receive antithymocyte globulin (ATG) and cyclosporin A (CyA) (G-CSF- group) or ATG, CyA, and G-CSF (G-CSF+ group). In the G-CSF+ group, the hematologic response rate at 6 months was higher (77% vs 57%; P = .03) than in the G-CSF- group. No differences were observed between the groups in terms of the incidence of infections and febrile episodes. There were no differences between the G-CSF- group and the G-CSF+ group in terms of survival (88% vs 94% at 4 years), and the development of myelodysplastic syndrome (MDS)/acute leukemia (AL) (1 patient vs 2 patients). However, the relapse rate was lower in the G-CSF+ group compared with the G-CSF- group (42% vs 15% at 4 years; P = .01). Further follow-up is required to elucidate the role of G-CSF in immunosuppressive therapy for adult SAA.