We present results of peripheral blood stem cell (PBSC) mobilization, collection, and follow-up from 3928 consecutive unrelated stem cell donors. Assessments were performed prospectively at baseline, leukapheresis, 1 month, 6 months, and annually after PBSC donation. During follow-up, side effects were recorded by return post questionnaires. The median CD34+ cell counts on day 5 were 67.5/microL in male and 51/microL in female donors. Bone pain and headache were the most common side effects of recombinant human granulocyte-colony stimulating factor. Central venous access was required for 23 donations (0.6%). Throughout the follow-up, the absolute neutrophil counts were slightly below the initial baseline values but remained within the normal range. The majority of the donors reported good or very good health. Malignancies occurred in 12 donors (0.3%), among whom were 1 case of acute myeloid leukemia, 1 case of chronic lymphatic leukemia, and 2 cases of Hodgkin disease. Only the incidence of Hodgkin lymphoma differed significantly from an age-adjusted population. In conclusion, 7.5 microg/kg per day lenograstim proved to be safe and effective for mobilizing hematopoietic stem cells for allogeneic transplantation. Long-term monitoring of healthy PBSC donors remains important to guarantee the safety standards of PBSC mobilization and collection.