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Objective: Assessment of factors that influence erythropoiesis during autologous blood donation. Design:Retrospective study. Setting:Autologous blood donation program at the central laboratory of a teaching hospital. Patients:All consecutive patients with initial hemoglobin concentrations of at least 11.5 g/dl and without acute cardiovascular or infectious diseases who planned to donate autologous blood prior to hip replacement surgery. Interventions: a) Between 1992 and 1997, up to 4 units of 500 ml blood each were withdrawn at weekly intervals within a 21-day period. Starting in January 1996, female patients were offered a more flexible donation scheme extended by 1 week, allowing to omit at least one of the 5 appointments depending on hemoglobin concentrations. b) Iron was substituted by oral (March 1992 to January 1995 and November 1995 to April 1997 2 ×?100 mg elemental iron/day) or intravenous routes (January 1995 to June 1995 200 mg elemental iron, April 1995 to September 1995 125 mg elemental iron, September 1995 to November 1995 62.5 mg elemental iron; each dose in an infusion of 500 ml 0.9% NaCl solution after blood donation). c) Based on hemoglobin measurements and body weight, the volumes of donated and regenerated packed red blood cells (RBCs) were calculated for each donor and statistically related to possible factors of influence. Results:In 215 women and 138 men, the mean total volumes of packed RBCs increased by 390 ml and 500 ml, respectively. Higher initial hemoglobin concentrations were significantly associated with the regeneration of greater RBC volumes. Because of a longer donation period, regenerated RBC volumes were higher in 79 women with 5 than in 80 women with 4 appointments (425 ± 173 ml vs. 350 ± 145 ml). In women only, younger age supported a faster RBC regeneration. Associations between erythropoietic regeneration and the form or dose of iron supplementation, reticulocyte counts as well as ferritin and erythropoietin levels were statistically not significant. Conclusions:Initial hemoglobin concentrations and the length of time from withdrawal of the first blood unit till surgery independently correlated with the volume of regenerated RBCs during autologous blood donation. In order to optimize the process of autologous blood donation and facilitate RBC regeneration, a suitable number of blood units should be donated before surgery as early as possible, but a minimum hemoglobin level during the preoperative period must be maintained. Other factors did not significantly correlate with RBC regeneration.