Administration of the myeloid growth factor G-CSF after allogeneic hematopoietic stem cell transplantation is usually well tolerated, and associated with rapid hematopoietic engraftment. We report a high incidence (50%) of side-effects associated with post-transplant G-CSF in patients with chronic phase chronic myeloid leukemia undergoing allogeneic HLA-identical sibling peripheral blood stem cell transplantation. One or more of the following signs and symptoms were observed shortly after the subcutaneous injection of G-CSF: dyspnea, chest pain, nausea, hypoxemia, diaphoresis, anaphylaxis, syncope and flushing. These reactions led to discontinuation of G-CSF in the majority of patients. Predictive factors could not be identified, and the underlying mechanism leading to these reactions is unknown.