Background and Objectives G‐CSF‐mobilized peripheral blood stem cells have long replaced marrow as the major source for allogeneic transplants. Conclusive evidence questioning the long‐term safety of G‐CSF for donors has not been provided, but the cumulative number of followed donors remains insufficient to rule out rare adverse events. A long‐term active follow‐up study of G‐CSF‐mobilized healthy volunteer donors was therefore performed.Patients and Methods Two hundred and three successive donors were evaluated pre‐apheresis, subjected to G‐CSF‐mobilization/apheresis, and actively followed for 5 years by the same physicians and laboratories. Follow‐up laboratory work included standard biochemical/haematological tests and T‐cell phenotyping.Results Donor epidemiology was typical for reported stem cell donor cohorts. Acute adverse effects of G‐CSF and apheresis were mild and transient, consistent with the previous reports. Mean circulating CD34+ cells after nine doses of G‐CSF were 124 per μl. Other biochemical/haematological parameters were also altered, consistent with G‐CSF treatment. Spleen enlargement was modest. At first follow‐up, all clinical and laboratory parameters had normalized. Leucocyte/lymphocyte counts and CD4/CD8 ratios were the same as during premobilization work‐up and remained unchanged throughout. A single severe but likely unrelated adverse event, a case of papillary thyroid carcinoma, was reported.Conclusion The studies add an observation time of almost 500 donor years to the growing body of evidence of the long‐term safety of G‐CSF for allogeneic donor stem cell mobilization.