A prospective study of 34 patients undergoing total hip replacement was done to determine whether homologous erythrocyte transfusion causes postoperative immunosuppression. In the transfused patient group (14 patients), there was a reduction in CD3+ and CD3+4+ cell numbers at Day 2, returning to preoperative levels by Day 7. In contrast, in the untransfused patient group (20 patients), there was no significant depression in these lymphocyte subgroups at Day 2 and an increase in total lymphocyte, CD3+, CD3+4+, and CD3+4-8- cell numbers at Day 7. In both patient groups there was postoperative leukocytosis, granulocytosis, and monocytosis at Days 2 and 7, with no significant change in postoperative B (CD19+) cell numbers, natural killer cells, or the minor T cell populations of CD3+4+8+ and CD3+25+. The lymphocyte functional test of Candida recall was significantly impaired at Day 7 in the transfused patient group, where Candida recall is a memory T cell response to an antigen extracted from the yeast Candida Albicans. These findings suggest that homologous erythrocyte transfusion after hip replacement surgery causes cell mediated immune suppression. The main clinical implication of the current study is that perioperative homologous erythrocyte transfusion may place patients at greater risk of infectious complications, including infection of the prosthesis.